338 THE ANATOMY OF THE HORSE. 



• 



The Hiatus Aorticus. — This is the opening between the right and left 

 pillars, close to the spine. It gives passage to the posterior aorta, and 

 to the initial portions of the thoracic duct and great azygos vein. 



Between the cms and the psoas parvus on each side the gangliated 

 cord of the sympathetic passes, and a little outward the great splanchnic 

 nerve passes between the same muscle and the edge of the diaphragm. 

 The asternal vessels penetrate the edge of the diaphragm at the 9th 

 chondro-costal joint. 



Action of the Diaphragm. — The diaphragm is the principal muscle of 

 inspiration. When it contracts it moves backwards, and thus increases 

 the antero-posterior diameter of the thorax. In this action it pushes 

 back the abdominal viscera, and causes the abdominal wall to descend. 

 The movements of the diaphragm affect principally its periphery, any 

 great backward movement of the tendinous centre being prevented by 

 the posterior vena cava, which passes like a ligament between the centre 

 and the heart. 



STRUCTURE OF THE STOMACH. 



Directions. — If only one stomach is available, inflate it, and ligature 

 the duodenum. Study first the serous coat, and then the longitudinal 

 and circular strata of muscular fibres, after which the ligature should 

 be removed from the duodenum, and the stomach everted and again 

 inflated. On the everted organ examine first the mucous membrane, 

 and then carefully dissect that and the submucous coat off, so as to 

 expose the inner or oblique layer of muscular fibres, and in particular 

 the arrangement of these and the middle or circular fibres around the 

 oesophageal orifice. It will be a great advantage, however, to have two 

 stomachs for dissection, using the first for the study of the peritoneal 

 and muscular coats, and the other for the examination of the mucous 

 membrane and interior of the stomach. 



The wall of the stomach comprises four layers, viz., serous, muscular, 

 submucous, and mucous. 



1. The Serous Coat is a smooth, glistening covering derived from the 

 peritoneum. It is united to the subjacent muscular coat by areolar 

 tissue, sometimes termed the subserous coat. 



2. The Muscular Coat. — The muscular fibres are disposed in three 

 planes, viz., an outer longitudinal, a middle circular, and an inner 

 oblique layer. Of these the circular layer is found all over the organ, 

 but the other layers are mainly confined to the left half. The fibres 

 are of the non-striped variety. 



The longitudinal stratum underlies the peritoneum, and it is the 

 least complete. Some of its fibres are continued from the oesophagus, 

 and others are proper to the stomach wall. They are best developed 

 on both faces of the left sac, and along the lesser curvature. A few 



