588 SPECIAL PHYSIOLOGY. 



folded in different birds, the convolutions being either spiral, concentric, or ir- 

 regular. The mucous membrane is usually plicated. The distinction between 

 small and large intestine now becomes less marked, there being no ileo-caecal 

 valve, and villi being found on the mucous membrane of both. Their place 

 of junction is, however, frequently indicated by the presence of a caecum, or 

 rather of two caeca, for this diverticulum is most commonly double. The cseca 

 are wanting in some vultures, in the cormorant, wryneck, and toucan, and in 

 many carnivorous, insectivorous, and frugivorous birds ; they are small and 

 short in other vultures, in the eagles, and in the solan-goose, and also, when 

 they exist, in the Insessorial tribes. They are longer in the nocturnal than 

 in the diurnal birds of prey. Amongst the Easores, they are short in the 



Eigeons, but very long in the grouse, each measuring three feet, or thrice the 

 jngth of the body, their internal surface being increased by eight longitudinal 

 folds ; in other Easores, they are of moderate length. In the Cursorial birds, 

 the intestinal canal, as well as other parts, approaches more nearly the mam- 

 malian character. The caeca, however, are absent in the cassowary, which 

 obtains a constant supply of succulent vegetable food ; whilst in the ostrich, 

 which lives upon dry and scarce food, they are wide, about two feet long, and 

 have an internal spiral fold, like that of the hare. In Birds generally, as in 

 Mammalia, the caeca are absent, or small, when the food is concentrated and 

 easily digestible ; but when it is slower of digestion, or is taken in larger quan- 

 tity, and at longer intervals, these appendages are most developed. 



The large intestine beyond the caeca is long and mammalian-like in the os- 

 trich, but usually is relatively short, straight, and not very wide ; it termi- 

 nates by an imperfectly valved circular opening in the dilated cavity called 

 the cloaca, into which also the ureters and the duct or ducts of the reproduc- 

 tive organs open. In the hinder wall of the cloaca is situated the glandular 

 organ known as the bursa Fabricii. 



Lastly, there exists in many birds a short, narrow, blind diverticulum, con- 

 nected with the small intestine ; this is the vestige of the vitelline duct, which, 

 in the embryo and young bird, connects the yolk sac with the intestine. It 

 is called the vitelline caecum ; it has no special digestive function. A similar 

 diverticulum is occasionally found in Mammalia, and even in Man. 



Reptiles. The alimentary canal in this Class is more simple than in Birds, 

 to which, however, it approaches more nearly than to that of Fishes. The 

 oesophagus varies in length, according to that of the neck ; it is wide, pli- 

 cated, and dilatable in the Ophidia ; as in Birds, it joins the stomach without 

 any constriction or cardiac orifice ; but the mucous membrane suddenly ceases 

 to have a dense epithelium, and becomes soft, smooth, and glandular. In 

 the larger Saurians, the first part of the stomach has the form and structure 

 of a gizzard, presenting thick muscular walls, the fibres of which radiate from 

 two opposite central tendons ; the pyloric part, more decidedly glandular, 

 corresponds with the short portion sometimes found between the gizzard and 

 the duodenum in Birds. In the Serpents, the cardiac part of the stomach is 

 long, slightly saccular, and highly dilatable, whilst the pyloric portion is nar- 

 rower and very muscular, being the only part like a gizzard. In the Chelo- 

 nians, the stomach is curved, and larger at the cardiac than at the pyloric end. 

 The pyloric valve is usually present in Eeptiles, though not very distinct, and 

 sometimes is scarcely recognizable. 



The intestine in Eeptiles is shorter and wider than in Birds. In the Sau- 

 rians, there is mostly an ileo-colic valve ; the crocodiles have no caecum. In 

 the Chelonians, the intestine is long and muscular ; an ileo-caecal valve usu- 

 ally exists, and also frequently a caecum. In Serpents, the small intestine 

 especially, is elongated ; the ileo-colic valve is indistinct, or its place is indi- 

 cated only by a change in the size of the canal ; the large intestine sometimes 

 has transverse folds in its interior, analogous with the spiral valve in the 

 same part in Cartilaginous fishes. As shown by the form of certain reptilian 

 coprolites, these folds must have been well developed in some extinct Saurians. 

 The mucous membrane of the large, as well as of the small intestine, is pli- 

 cated and villous. The lower end of the larger intestine forms a cloaca, re- 

 ceiving the ducts of the urinary and reproductive organs. The presence of a 

 caecum in certain Chelonia furnishes an additional example of the association 



