THE VASCULAR SYSTEM. 289 



stomach is, as a rule, a simple transversely placed sac, but is 

 frequently divided by the gradual differentiation and constriction of 

 its anterior, lateral, and posterior regions into a number of parts, 

 which are most completely separated in the Ruminants and distin- 

 guished as four separate stomachs. The pyloric region is principally 

 distinguished by the presence of gastric glands, and is more or less 

 sharply separated from the beginning of the small intestine by a 

 sphincter muscle and by an inwardly projecting fold (pyloric valve) . 

 The intestine is divided into a small and a large intestine, the 

 boundary between which is indicated by the presence of a valve and 

 a caecum, which is especially developed in herbivorous animals. The 

 anterior part of the small intestine, or duodenum, contains the so- 

 called Brunner's glands in its mucous membrane, and receives the 

 secretion of the large liver and of the pancreas. The liver is multi- \/ 

 lobed, and is sometimes without a gall bladder. When a gall bladder 

 is present the bile duct (d. cysticus), and the hepatic duct (d. hepaticus) 

 unite to form a common duct (d. clioledochus). The small intestine is 

 longest in animals which eat grasses and leaves, and is characterised 

 by the numerous folds (valvulce conniventes) and villi of its mucous 

 membrane, and by the possession of a great number of groups of 

 glands (Lieberkiihn's, Peyer's glands). The terminal region of the 

 large intestine or rectum opens, except in the Monotremata which 

 are characterised by the possession of a cloaca, behind the urogenital 

 opening, though the two openings are sometimes surrounded by a 

 common sphincter (Marsupialia). 



The heart (fig. 676) of Mammalia, like that of Birds, is divided 

 into a right venous and a left arterial portion, each with a ventricle 

 and auricle (sometimes as in Halicore the division is marked exter- 

 nally). It is enclosed in a pericardium, and sends off ac arterial 

 trunk, which forms a left aortic arch,from which two vessels frequently 

 arise, viz., (1) a right anonyma, with the two carotids and right 

 subclavian ; and (2) the left subclavian ; or, as in man, three vascular 

 trunks, viz., (1) a right anonyma, with the right carotid and right 

 subclavian ; (2) the left carotid ; and (3) the left subclavian, all close 

 to one another. As a rule, a superior and an inferior vena cava open 

 into the right auricle ; more rarely, as in the Rodents, Monotremes, / 

 and Elephants, there are two superior vena3 cavse. Retia mirabilia 

 have been recognised principally for the arterial vessels, and are 

 found on the extremities of burrowing and climbing animals (Stenops, 

 Myrmecophaga, Brady pus, etc.) ; on the carotids round the hypo- 

 physis, and on the ophthalmic arteries in the orbit in Ruminants ; 



VOL. II. 19 



