THORAX AND ABDOMEN OF THE HORSE 131 



artery to the spermatic cord and testis of the male, or the ovary, uterine 

 tube, and part of the uterus of the female. 



Dissection. — The stomach, duodenum, spleen and liver must now be 

 removed from the abdomen, and their structure studied as far as is 

 possible with the naked eye. 



Structure of the stomach. — In common with the other parts of 

 the alimentary canal contained within the abdomen, the wall of the 

 stomach is composed of four layers — (1) an external serous tunic derived 

 from the peritoneum ; (2) a muscular tunic ; (3) submucous tissue ; and 

 (4) a mucous lining. 



The serous tunic is closely applied to the muscular tunic over the 

 greater part of the stomach. There is a very narrow strip uncovered 

 by it along the greater curvature where the greater omentum and the 

 gastro-splenic ligament are attached : and a wider area along the lesser 

 curvature and in association with the attachment of the gastro-phrenic 

 ligament. In the region of the lesser curvature the peritoneum is 

 notably strengthened by elastic tissue. 



The muscular tissue is disposed in three strata. When the serous 

 investment of the stomach has been removed, longitudinal fibres, most 

 numerous along the curvatures but very sparse elsewhere except at the 

 antrum pylori, are exposed. There should be little difficulty in 

 determining that these fibres are continuous with the longitudinal 

 fibres of the oesophagus and duodenum. A stratum of circular fibres is 

 not readily defined over the left half of the stomach, where it is replaced 

 by oblique fibres, but can be distinguished easily over the right half of 

 the organ, where its thickness increases as the pylorus is approached. 

 At the pylorus itself the circular fibres are numerous and produce a 

 double sphincter {sphincter pylori). The circular fibres of the stomach 

 are to be looked upon as a continuation of the circular fibres of the 

 oesophagus, and are clearly continuous with those of the duodenum. 



The deepest fibres, oblique in direction, are best examined from 

 within after the mucous membrane has been studied and removed. If 

 possible the stomach should be turned outside in ; but, if the preserva- 

 tive has rendered this impossible otherwise, it may be affected through 

 a small opening made along the greater curvature. 



The mucous membrane of the stomach of the horse is remarkable, in 

 that it is divided into two parts, very different in character, which meet 

 at an abrupt uneven line, the margo plicatus. This line corresponds to 

 a shallow constriction that has already been noticed on the exterior. 

 The membrane to the left of the line is pale, dense and skin-like in 

 appearance. Indeed, it closely resembles the mucous lining of the 



