266 COMPARATIVE ANATOMY 



upon as parts of one and the same chamber, simply serve as storage 

 cavities, the food returning from them into the mouth, once more 

 to undergo mastication. It then passes into the psalterium, and 

 finally into the abomasum, the latter alone being provided with 

 gastric (rennet) glands, and serving as the true digestive stomach. 

 The psalterium "is the latest to be differentiated both phylo- and 

 ontogenetically, and is rudimentary in the Tragulidae. In Camels 

 the rumen gives rise to two masses of gland-containing outgrowths, 

 known as " water-cells." In the Cetacea (Fig. 212, c and D), Hippo- 

 potamus, and Brad ypus, the stomach is divided into several chambers, 

 and various other modifications in form and structure are met with 

 amongst Mammals. Thus in the Kangaroo, for instance (Fig. 212, 

 b), the walls of the stomach are curiously folded. 



The small intestine is usually long, and varies more as to rela- 

 tive length and diameter in domesticated than in wild forms. 



The large intestine, which is made up of a varying number 

 of coils, usually reaches a great length, and its diameter is 

 much greater than that of the small intestine : these two por- 

 tions are thus sharply marked off from one another, and the 

 distinction between them is rendered still more marked by the 

 sacculations of the anterior part of the large intestine. Only the 

 posterior portion of the latter, or rectum, which passes into the 

 pelvic cavity, corresponds to the large intestine of lower Verte- 

 brates ; the remaining and far larger part occurs only in Mammals, 

 and is called the colon. 



The csecum, which is almost always present, undergoes various 

 modifications both as to form and size. Thus in Edentates (Manis, 

 Bradypus), Carnivora, Odontoceti, Insectivora, and Cheiroptera, 

 it is very small or even entirely wanting, while in Herbivora 

 it may exceed the whole body in length. An inverse development 

 in size is usually noticeable between it and the rest of the large 

 intestine. In many cases (many Rodents, Monkeys, and Man) an 

 arrest of a portion of the csecum takes place in the course of 

 individual development, so that little more than the distal end 

 {^processus venniformis) remains (Fig. 190). In Lepus the enor- 

 mous caecum is provided with a spiral valve ; and in Hyrax, besides 

 a large sacculated C£Ecum at the junction of the small and large 

 intestines, there is a pair of large, simple, conical caeca further back. 



Monotremes only amongst Mammals possess a distinct cloaca, 

 though in Marsupials and some Rodents the anal and urinogenital 

 apertures are surrounded by a common sphincter. In other 

 Mammals these apertures become completely separated from one 

 another. 



