Abdomen of the horse

Internal Regional Anatomy

In the horse the stomach, even when distended with food, is relatively small; it is situated mainly to the left of the median plane of the body, in the concavity of the diaphragm immediately behind the liver. The major portion of the small intestine, with the exception of the duodenum and terminal segment of the ileum, is supported by relatively long folds of mesentery, so that its loops vary greatly in position; the major part is usually situated in contact with the abdominal wall on the left side, a few coils may reach the floor of the abdomen. The supporting mesentery originates from the roof of the abdomen beneath the first and second lumbar vertebrae. At no point is it possible to recognize small intestine by palpation through the abdominal wall. The various parts of the large intestine, comprising the caecum, the large colon, the small colon and the rectum, with the exception of the small colon which has a long supporting mesentery, are usu­ally found to occupy a relatively static position. The rounded base of the caecum is situated in the right iliac and sublumbar region while the comma-shaped body which extends downwards and forwards, almost entirely fills the right flank area. The blind apex of the caecum reaches the abdominal floor to the right of the median plane, behind the xiphoid cartilage. The ileocaecal ori­fice is situated on the anterior concave curvature of the base, with the caecocolic orifice placed about 5 cm away in a lateral direction.

The first part of the large colon, the right ventral colon, originates from the concave base of the caecum opposite the lower extremity of the last rib or intercostal space. It then passes down­wards and forwards, contiguous to the right costal arch, and continues along the floor of the abdomen where, above the xiphoid cartilage, it turns medially and then backwards on the abdominal floor, on the left of the right ventral colon, until it reaches the pelvic inlet. Here it bends acutely upwards and forwards, forming the pelvic flexure, and continues as the left dorsal colon, the lumen of which is greatly reduced in size compared with that of the two ventral segments. The left dorsal colon is imme­diately dorsal to the left ventral segment; it passes somewhat downwards and forwards op­posite the middle section of the left flank to run medial to the costal arch, and reach the dia­phragm and left lobe of the liver, where it bends medially to the right, and then backwards to form the diaphragmatic flexure. The terminal part, the right dorsal colon, extends backwards dorsal to the right ventral segment, until it reaches the medial aspect of the caecum where it bends to the left and dorsally, to a point below the left kidney, where it becomes reduced in diameter and becomes confluent with the small colon.

The small colon, which is about 3-3,5 m in length, commences behind the stomach, ventral to the left kidney, where it is attached in the sublumbar region by a long mesentery. The coils lie mainly between the stomach and pelvic inlet, dorsal to the left segments of the large colon, in which situation they mingle with loops of small intestine, and make contact with the abdominal wall in the left flank area. The small colon con­tinues as the rectum at the pelvic inlet. The rectum, which is about 30 cm in length, is often situated slightly to the left of the median plane in the pelvis, where it is related anteriorly to the pelvic flexure of the large colon, and loops of the small colon. The retroperitoneal part of the rectum is related dorsally and laterally to the pelvic wall. Ventrally, in the male, it is in contact with the urinary bladder, the seminal vesicles, the terminations of the vasa deferentes, the prostate gland, the bulbourethral glands and the pelvic portion of the urethra. In the female this relationship is with the uterus, vagina and vulva.

The structurally important features of the gastrointestinal tract in the horse are the rela­tively small size of the stomach, the situation of the entry and exit orifices of the double blind-ended caecum, the sudden reduction in the diameter at two points in the large intestine and, in the case of the last, the longitudinal muscle bands in the wall of the caecum and colon. This feature can be recognized during rectal examination, particularly in relation to the sacculated character of both the large and small colon.

Digestion in the Horse

In the adult horse the stomach accounts for 12% of the total capacity of the digestive tract. Under normal feeding practices the stomach is never completely empty and secretion of acid gastric juice occurs continuously. Emptying of the stomach normally commences very soon after the horse begins to feed due to the development of powerful peristaltic contractions, probably initiated by vagal stimulation, which ensure the rapid onward passage of the ingesta from the stomach to the small intestine and into the large intestine. When eating stops the stomach con­tractions cease.

The precise mode by which the stomach func­tions in digestion is not conclusive; it has been suggested that maceration and bacterial break­down, particularly of food with a high fibre content, may be of greater significance than enzymic digestion. The appearance of significant amounts of lactic and butyric acids in the stomach, when a hay diet is fed, favours this view. Enzymic action in the small intestine is responsible for the elaboration of protein and soluble carbohydrate which are absorbed before they can be subjected to attack by the flora of the large intestine. When the ingesta reaches the large intestine it is subjected to fermentation reactions by the microflora with the production of volatile fatty acids which are absorbed. Water and electrolytes are also absorbed, mainly from the colon. The motility of the large intestine in the horse is more frequent than in other species.

Clinical Examination of the Stomach and Intestines

The position of the stomach in the concavity of the diaphragm at the anterior part of the abdomen, where because of its relatively small size it does not come into contact with the abdominal wall, except to a limited extent in certain circumstances, causes difficulty in the diagnosis of gastric disorders in the horse. In considering the possibility of gastric disease the clinician must of necessity, therefore, take into account all the available indirect evidence that can be obtained.

Dilatation of the stomach in the horse is accompanied by clinical signs indicating acute, continuous pain consisting of elevated temperature, increased pulse frequency and, because of voluntarily restricted diaphragm movement, shallow breathing, with congestion of the mucous membranes. The intensity of the pain reaction in the individual animal is indicated by the degree of sweating, which is usually profuse. When gastric distension is severe, the horse assumes the dog-sitting posture in an attempt to relieve pressure on the diaphragm.

Gastric dilatation is caused by impaction, «xcessive gas production or consumption of excessive quantities of fluid, e.g. cold water, whey, etc. The most common cause of gastric impaction is unrestricted feeding on ground grains such as wheat, barley, cornmeal, etc. Dila­tation as a complication of impaction may follow pyloric obstruction due to compression by a neoplasm or stricture of the pylorus. Excessive gas production in the stomach commonly fol­lows the ingestion of succulent, rapidly fermen­table foods such as growing oats, corn and legumes. Consumption of excessive amounts of cold water is more likely to occur when the horse is fresh from work and likely to be overheated. Dilatation in such cases is usually of temporary duration due to the low level of intestinal moti­lity and the tendency to pyloric spasm.

Gastric tympany, occurring without intestinal flatulence, may be associated with tympanic resonance in the anterior abdomen on the left side. In this event, impaired intestinal motility is indicated by early reduction, followed by com­plete cessation, of defaecation. Periodic eructa­tion of gas may be observed, and passage of the stomach tube permits the escape of large quan­tities of gas. When fluid causes distension of the stomach, regurgitation of small amounts of food admixed with liquid occurs, followed by its passage down the nasal cavities, which causes irritation, snorting and coughing. In this situa­tion, fluid escapes in large amounts when the stomach tube is passed. When the stomach is impacted with food, eructation of gas or fluid does not occur, and introduction of the stomach tube does not lead to the return of any of the gastric contents. Here, occasional vigorous eructation movements occur which coincide with periodic bouts of intense pain and discomfort.

When eructation is associated with evacuation of fluid or gas from the stomach, the animal obtains relief, the pain reactions subsiding until gastric distension recurs. A check test which reveals the presence of hydrochloric acid will confirm that the regurgitated material originated in the stomach. In grass sickness the reaction of the mainly fluid vomitus is usually alkaline. If gastric distension persists for some hours, exhaustion and toxaemia will reduce the inten­sity of the animal's reaction to pain and, super­ficially, it may appear that clinical improvement has occurred. Closer examination will reveal that the pulse frequency has increased with con­siderable deterioration in pulse quality, the general reflex and toxaemic effect on the vascular system being revealed by the cold clammy state of the body surface.

If, following a severe bout of pain with retch­ing, terminating in free vomiting, the horse appears to show sudden relief of pain, the likeli­hood that rupture of the stomach has occurred should be considered. In this event retching ceases and the animal stands quietly, but very soon signs of shock appear, consisting of tremors, cold sweating, subnormal temperature, severely congested mucous membranes with a pulse rate in excess of 120/minute and having running-down characteristics. Auscultation of the abdo­men will reveal that peristaltic activity has ceased.

Gross distension of the stomach causes the spleen to become displaced medially and pos­teriorly within the abdominal cavity. The degree of displacement may be sufficiently great to permit the sharp, firm posterior border of the spleen to be readily recognized during rectal exploration. In small horses, the swollen mass of the stomach may be palpated just below the roof of the abdomen, near the kidneys. When rupture of the stomach has occurred the visceral peritoneum will have a dry, roughened feel, because of the presence of ingesta and develop­ing peritonitis. In this condition manual explora­tion will be readily performed because of the flaccid state of the intestines.

The proportionately small size of the stomach in the horse, relative to the food requirements of the animal, imposes greater demands for intesti­nal efficiency. This is provided to some degree by the anatomical complexity of the large intes­tine. Because of this situation the incidence of intestinal maladies is quite high. The most im­portant of these include spasm, tympany, im­paction and inflammation or enteritis, in all of which distension and modifications of motility occur. As in the case of the stomach, the distension may be static when there is an accumu­lation of ingesta, gas or fluid, or transient when it is the result of periodic, segmental spasm and hypermotility. These several conditions will be associated with abnormal behaviour typical of the colic syndrome, although the intensity and character of the pain manifestations are related to the severity of the malady. In the more acute states, e.g. tympany, enteritis, acute obstruction, etc., systemic effects (shock, dehydration) will influence the character of the pulse and respira­tions. To aid understanding of the basic situa­tion, static distensions are classified as causing physical colic, and transient distensions as caus­ing functional colic.

The modifications in intestinal motility consist of increased or decreased function. Hypermoti­lity, characterized by diarrhoea, occurs in enter­itis and peritonitis. The causes of enteritis in horses are many and varied. The young foal is similar to neonatal animals of other spcies in being highly susceptible to bacterial, protozoan, mycotic and possibly even viral infections which are directly or indirectly responsible for intesti­nal inflammation. The more common diseases include colibacillosis, salmonellosis, shigellosis, aspergillosis, equine viral arteritis (in a propor­tion of cases), 'colitis-X' (the cause of which is as yet unidentified), strongylosis, ascariasis, etc. Chemical agents and poisonous plants may occa­sionally cause intestinal irritation, as also may physical agents such as sand or soil. Peritonitis develops as the result of penetrating wounds of the abdominal wall which may arise either accidentally, in the course of hunting, riding or working in harness, or intentionally as for instance following intra-abdominal surgical pro­cedures or trocarization of the caecum or colon. Perforation of gastric ulcers caused by the larvae of Gasterophilus spp. or Habronema megastomum is an occasional cause of peritonitis. In con­ditions such as gastric rupture, intestinal torsion or intussusception, mating accidents and over-vigorous rectal examination, acute fatal perito­nitis develops. Death, however, is usually the result of shock and in some cases internal haemorrhage.

Reduction in intestinal motility may develop gradually, as in impaction of the large or small colon or the caecum. In intestinal obstruction caused by torsion, intussusception or incarcera­tion, there is sudden, virtually complete cessation of motility. Impaction of the ileocaecal valve is associated with an initial period of hypermotility of about 12 hours duration, followed by complete cessation of peristalsis.

Inspection of the abdomen will reveal general distension in primary tympany, which is usually the result of recent feeding on rapidly fermen­table, lush green food. In secondary tympany, which develops following acute intestinal ob­struction, the segmental distension is usually insufficient to give rise to an externally recog­nizable increase in the size of the abdomen. External palpation is unlikely to reveal clinical evidence of any great value, apart from pain and tension, in respect of the abdomen in the horse. Percussion will indicate the presence of tympany, the increased resonance being general through­out the area in the primary form, and localized in cases of secondary origin. Segmental intestinal impaction is, however, not invariably revealed because of the uncertainty of eliciting a signifi­cantly dull percussion note at the appropriate point on the abdominal wall.

Auscultation of the abdomen in the horse is of considerable value because it will provide an opportunity to assess the degree of functional activity of the intestines from the peristaltic sounds. Loud gurgling or rumbling sounds suggest enteritis, spasmodic colic or the early stages of acute obstruction arising from volvulus, intussusception, strangulation or impaction of the ileocaecal valve. Peristaltic sounds are re­duced, or absent, in impaction, atony and after a few hours in acute obstruction. Tinkling sounds are heard when there is general intestinal tympany or local retention of gas, because of segmental spasm. The quality of the intestinal sounds is also related to the character of the diet, being loudest when the food contains relatively large amounts of water. Loud bowel sounds may reduce the audibility of the functional sounds produced by other organs such as the lungs. In rupture of the diaphragm, when segments of intestine pass forwards into a pleural sac, peri­staltic sounds can be detected during ausculta­tion over the lower part of the respiratory area on the affected side. The degree of intestinal motility is reflected in the quantity of faeces, the character of which will provide further evidence of clinical value.

Rectal Examination

Prior to commencing the physical procedures involved in rectal examination the condition of the anal region is considered, noting the presence of neoplasms (melanosarcomas), or greyish-white deposits, signifying infestation with Oxyuris equi (because of the intense irritation caused by the parasite loss of hair at the base of the tail is a usual feature), and the tone of the anal sphincter, which is reduced in old age and in paralysis of the rectum.

When the hand reaches the ampulla of the rectum the quantity of faeces present should be assessed, also the tone of the musculature; bal­looning or contraction of the rectum, paralysis of the rectum (most common in the mare approa­ching parturition, occasionally in encephalitis and in some cases of paralysis of the tail) and laceration and perforation of the wall can be recognized. Traumatic injury to the rectum, resulting from injudicious rectal exploration, sadism or in mares from mis-service or accident during foaling (rectovaginal fistula), if extensive, is readily recognized, and even where the damage is slight, or of recent origin, it is possible to detect a break forming a depression of the normally smooth mucous membrane, and which contains blood and faeces. The extent and the depth of the injury are determined by careful digital exploration, noting whether the finger, or the whole hand, can be introduced into the wound or, in some cases of perforation, even into the peritoneal cavity where segments of intestine are directly palpable. The condition of the pelvic bones and sacrum can be determined by palpa­tion. Fracture of the pelvic bones may be recog­nized by hearing and feeling the grating (crepi­tation) of the opposing parts of the damaged bone during movement (over-riding), induced by an assistant rocking the hindquarters of the animal from side to side. The presence of calculi in the bladder or of neoplasms involving this and other organs and tissues in the pelvic cavity is easily determined.

Only a few parts of the normal intestines can be readily identified by palpation during rectal exploration. Definition is possible, however, when a portion of intestine is distended with gas or is the site of obstruction as in impaction, intussusception, volvulus or incarceration, pro­vided the affected part is palpable.

The small colon is disposed in irregular loops distributed across the posterior part of the abdo­minal cavity and may even be found within the pelvic cavity. Almost consistently a segment of small colon can be palpated in front of, and below, the brim of the pelvis; it is as thick as the forearm of a man and usually contains balls of faeces (balled faeces occur only in the small colon and rectum), and is also recognizable because of the longitudinal muscle bands and sacculations of the wall. It can be grasped and moved easily in all directions on account of the long supporting mesentery. In impaction of the small colon, the offending faecal mass, which is very firm, varies in size from that of a turkey egg to that of a large coconut, depending to some extent on the size of the animal. Careful explora­tion and assessment may be required before the nature and situation of the condition, which is readily confused with impaction of the ileocaecal valve, are recognized. The character of the cli­nical signs and course of the disease will be of great significance.

In many horses, about 40-60 cm of the left ventral and the left dorsal sections of the large colon, together with the pelvic flexure, are easily palpated and recognized. These sections of the large intestine are abridged by a fold of mesen­tery varying from 5 cm to almost 15 cm wide at the pelvic flexure, but the united segments lie freely in the abdominal cavity. Although their normal position is along the left side, in contact with the abdominal wall, extending back to the pelvic inlet, these left divisions of the large colon may be situated medially, or even on the right of the median plane. The ventral portion differs considerably in certain anatomical features from the dorsal part, the former is about 20-30 cm in diameter, and its surface bears narrow, longitu­dinal, ribbon-like thickenings comprised of smooth muscle layers, and transverse constric­tions and projecting pouches (sacculations); the latter segment, on the other hand, is about 7-10 cm wide, its surface is smooth and only a single longitudinal muscle band is present in the vicinity of the attachment of the abridging mesenteric fold. The left dorsal colon is often situated slightly medial to the ventral division. The pelvic flexure and contiguous parts of the large colon can be readily grasped with the hand, in their normal situation towards the left side, just in front of the pelvic inlet. The transverse and right sections are not ordinarily palpable— they are out of reach of the hand. In rare instances, in medium to small horses, the stomach-like dilatation of the right dorsal seg­ment can be palpated, medially and anterior to the base of the caecum as a large balloon-like object, which can only be reached with the fingertips.

The commonest malady affecting the large colon is impaction with food residue or entero-liths. It commences in the left ventral colon and extends towards the sternal flexure. The impac­ted mass is firmest at the pelvic flexure which, in well established cases, is displaced posteriorly on the floor of the pelvis, and may be palpated immediately the fingers pass beyond the anal sphincter. In moderate impaction the mass can be indented by finger pressure, whereas in more severe cases the consistency is such that this is difficult to achieve. A horse affected with impac­tion of the large colon may stretch, or strain as if to relieve the condition, and may periodically adopt abnormal postures such as kneeling, 'dog-sitting' or standing with the hindquarters pushed into a corner. Torsion of the left seg­ments of the large colon is recognized by noting the absence of faeces in the rectum, gaseous distension of the intestines anterior to the point of occlusion, and the rotation of the pelvic flexure, which may occur either to the left or the right. In occasional cases these segments may be turned forwards on themselves, the displacement occupying a dorsal or ventral position, so that the pelvic flexure cannot be reached. In all forms of colonic displacement the clinical signs usually develop suddenly and are those associated with acute abdominal pain.

On the right side, in the upper anterior part of the abdominal cavity the immobile, rounded base of the caecum is accessible to the fingertips. In many instances, when in a normal state the caecum cannot be recognized; it is only readily identified when it is distended with solid or gaseous contents. Of the body of the caecum, only those parts contiguous to the base are normally palpable and then only readily identifiable when it is also distended. If the caecum is displaced and kinked backwards, its apex is sometimes within reach; it may even be found in the pelvic cavity. Distension of the caecum arises from impaction or tympany. General clinical signs are acute in gaseous distension, which usually involves neighbouring parts of the small and large intestine. The two conditions can be readily differentiated during rectal examination.

The terminal portion of the small intestine, which is distinguished anatomically from the remaining thin-walled parts of the intestine by reason of its thick muscular wall, can occasionally be palpated, more particularly when it is dis­tended. It is situated anteriorly in the upper area of the abdominal cavity, and it is recognizable as a firm tube 5-8 cm in diameter when normally distended, running horizontally, or obliquely, from left to right towards the base of the caecum. Towards the left it is mobile, near its termination with the caecum, on the right, it is fixed.

Impaction of the ileocaecal valve region, a not uncommon cause of colic in the horse, is indi­cated during rectal examination by the con­tracted state of the large intestine and the considerable distension of the small intestine by gas and fluid, which latter makes the examination difficult. This condition may be confused with volvulus of the small intestine because tightly stretched folds of mesentery may be palpable. Intestinal intussusception in the horse, a rela­tively rare condition seen most often in foals, often involves the ileum, more usually near its termination. In this case rectal examination reveals an empty rectum and reduction in the size of the large intestine, with a crepitant mass anterior to the brim of the pubes.

The more anterior parts of the abdominal cavity, which are within reach of the hand, are occupied by loops of small intestine, which during palpation give an impression of vague, indefinite objects which cannot be grasped. When distended with gas, the loops may then be more readily identified as resilient, cylindrical objects which are smooth, readily mobile and 7 cm or more in diameter. Gaseous distension is most marked if a segment of small intestine is the site of volvulus when, even if the affected part cannot be palpated, the folds of supporting mesentery, which are twisted into rope-like struc­tures extending downwards in varying directions from the roof of the abdomen, can be recog­nized.

Although not usually palpable during rectal exploration, the duodenum may be identified in some small horses. Even then it is only the terminal part that it is possible to palpate; it is recognized as a smooth-walled, compressible, pliant tube about 5-8 cm in diameter, emerging between the base of the caecum and the right abdominal wall, and passing to the left above the base of the caecum, in close contact with the dorsal wall of the abdomen and the right kidney, towards the anterior origin of the mesentery beneath the left kidney. It crosses the median plane ventral to the second or third lumbar vertebra.

The relative size and situation of the stomach in the horse ensures that it is not usually palpable. Exceptionally, when acutely dilated, it is within reach of the hand near the dorsal part of the abdomen. If palpable, the tense or firm sac-like character of the affected organ makes iden­tification an easy matter.

The spleen is not regularly palpable in normal horses. Its location is sought by advancing the hand, with the fingers extended, forwards from the pelvis along the left wall of the abdomen above the left dorsal segment of the large colon. The posterior border of the spleen approximates to the left costal arch, so that if the fingers reach this point they will pass between the abdominal wall and the spleen. When palpable the normal spleen is recognized by its usual, but not invariable, position in contact with the abdominal wall, its soft, yielding texture and its sharp, vertical, posterior border. The position of the spleen is influenced by the degree of distension of the stomach. When the stomach is dilated, the spleen is displaced medially and backwards. Segments of intestine may insinuate themselves between the spleen and the abdominal wall either from above, over the lienorenal fold of mesentery, or from below. In such circumstances the spleen is situa­ted away from the abdominal wall towards the median plane.

When enlarged the spleen may be more readily palpated during rectal examination. Splenic en­largement is most marked in congestive heart failure, portal obstruction or neoplastic involve­ment. In the last condition the disproportion may be local. Circumscribed enlargements of varying size also occur in tuberculosis. In general en­largement of the spleen the posterior border is thicker than normal.

The left kidney is situated well forward on the dorsal aspect of the abdomen beneath the last rib and transverse processes of the first two or three lumbar vertebrae, a little to the left of the median plane. It is usually held firmly in position by fascia and it is only rarely suspended by a mesenteric fold. Only the posterior pole of the left kidney is palpable, and then only in small horses. In such circumstances the fingertips impinge on a firm, semi-spherical body, some 15 cm in diameter, with a smooth surface. Very occasionally the renal artery can be recognized at the hilus. The right kidney is situated further forward than the left and is therefore out of reach. The ureter, in each case, can be identified only when it is grossly distended or thickened. A pain reaction to reasonable pressure on the kidney should be related to the general clinical picture in suspected cases of nephrosis or nephritis.

The abdominal aorta is identified as a strongly pulsating tube about 2-5 cm in diameter, situated under the vertebral column. It is located by introducing the arm into the rectum as far as the elbow, and then rotating the hand until the palm can be pushed upwards against the lumbar vertebrae and psoas muscles. By following the vessels backwards with the fingers, its division beneath the sixth lumbar vertebra into left and right external iliac, left and right internal iliac (hypogastric), and coccygeal arteries, is reached. Thickening and irregularity in the arterial wall are palpable over a variable distance in iliac thrombosis, which causes a mild or acute syn­drome with a corresponding degree of lameness.

The anterior mesenteric artery and some of its branches can be identified by tracing the abdo­minal aorta forwards; in the position of the first lumbar vertebra a pulsating vessel, running ver­tically downwards, is palpable. If affected with a verminous aneurysm, the arteries, at their origin, are thickened, have a rough, uneven surface and may be the site of a pain focus. The condition arises from the migration of the larvae of Strongylus vulgaris; it may be followed by secondary bacterial infection with Streptococcus equi, Actinobacillus equuli or Salmonella typhi-murium. In either case there is segmental atony of the large intestine which is expressed clinically by recurrent attacks of spasmodic colic, or per­sistent diarrhoea.

If, during palpation of either of the paired iliac arteries or the anterior mesenteric artery, a vibration thrill (fremitus) is found to replace the individual waves of the pulse, this indicates the presence of a thrombus (clot) in the artery con­cerned. In aneurysm there is an increase in the diameter of the artery, although this does not imply an increase in the size of the lumen in all cases.

The urinary bladder is palpated beneath the rectum, on the floor of the pelvis, by moving the palm of the hand backwards and forwards in the rectum. The empty bladder is recognized as a small, slightly yielding body, roughly the size and shape of a small coconut. When fully distended it is easily and distinctly identified as a fairly firm, globular, smooth-walled structure, the anterior extremity (vertex) of which projects from the pelvis to reach the abdominal floor in its posterior part. In urinary retention, caused by obstructive urolithiasis, the bladder may reach enormous dimensions. Cystic calculi, in the form of moderately large, firm objects, are usually easily identified by palpation of the bladder. Thickening of the bladder wall, which occurs in chronic cystitis, can also be appreciated.

The intrapelvic urethra in male horses can be palpated on the floor of the pelvis, immediately the fingers pass through the anus, as a firm, cylindrical structure about 2-5 cm or less in diameter which runs horizontally in an anterior direction. In urinary retention associated with pain, caused by cystitis, urethritis, calculi in the urethra and sebaceous concretions in the coronal fossa of the glans penis, palpation of the pelvic portion of the urethra produces spasmodic jerking of the penis.

In the stallion, a possibly important aspect of rectal examination concerns the abdominal in­guinal ring (more correctly the vaginal ring), which is located in the following way: the anterior border of the pubic bone is identified with the extended fingers; the palm of the hand is placed upon it and the fingers then flexed until their tips come into contact with the abdominal wall. If the abdominal wall is now palpated for a distance of about 15 cm by moving the hand to the left or right of the median plane, a slit-like opening about 8-15 cm long is located on each side extending forwards and laterally. It is usually wide enough to admit only the tips of one to three fingers. In addition, the spermatic cord, the constituent parts of which unite together at the vaginal ring, is distinctly palpable as a cord-like structure as thick as the thumb, run­ning downwards and outwards through the inguinal canal. If a segment of intestine has entered the inguinal canal and become incar­cerated in the tunica vaginalis sac, it will be recognized as a thick cord, traction on which causes a pain reaction.

In the mare, the uterus is palpable (beneath the hand) as a short body interposed between the rectum and bladder, and continued anteriorly by the diverging cornua which are slightly concave towards the dorsal aspect. Although the body of the uterus is situated directly beneath the rectum, partly in the pelvis and partly in the abdominal cavity, considerable experience is necessary in order to recognize it; the more usual procedure is to locate one of the ovaries and slide the hand under the anterior extremity of the correspond­ing uterine cornu, so that the whole uterus can then be systematically palpated. In early preg­nancy a bulge is recognizable in one horn to­wards the body; this increases in size as gestation progresses until the weight of the gravid organ is such that it is pulled down into the abdomen. Metritis causes a variable degree of general increase in the size of the uterus so that it is more readily identified.

The ovaries are located, one on each side, in the sublumbar region ventral to the fourth or fifth lumbar vertebra, in contact with the dorsal aspect of the abdominal cavity. They are recog­nizable because of their firm consistency, re­lationship to the uterus and relative immobility, the mesovarium being only 8-10 cm long. Their size is variable, being on average 5 cm long and 2-5 cm or more thick.

The lymph nodes of the abdominal cavity, including the iliac and lumbar groups, are palp­able when normal or when enlarged as variously sized, more or less spherical nodular bodies, situated in the appropriate position. Lymph node enlargement, in occasional instances, is caused by neoplastic states such as malignant lymphoma, which is multiple and involves lymph nodes in various parts of the body, including the abdomen, and melanoma which is most common in old grey horses, often originating in the perineal and perianal region, from where it metastasizes to the pelvic lymph nodes. A group of lymph nodes just within reach of the fingers can be palpated in the vicinity of the anterior mesenteric artery. In certain diseases (strangles) the group may be enlarged and painful (abscessation).

The peritoneum, both visceral and parietal surfaces, is palpable only in its posterior area. Normally it is smooth and not painful on palpa­tion. In inflammatory and neoplastic conditions, it becomes rough or nodular, and palpation causes pain. In rupture of the stomach or intes­tines, particles of food cause recognizable roughening of the peritoneum, or emphysema can be identified. The mesenteries are the site for rare cases of lipoma which, because they cause no obvious clinical signs although they are in most cases multiple, are usually only diagnosed on post mortem examination. Individual tumour masses are pedunculated and vary in size.