STOMACH AND INTESTINE. 



.301 



In the BatracJnan division, however, the 

 separation into these two segments is some- 

 times absent. While sometimes, as in the Toad 

 and Frog, there is a distinct large intestine, 

 into which the smaller portion opens laterally, 

 so as to form a caecum. In the former of 

 these two genera there is no valve. 



In the Ophidian the two portions are 

 generally distinct and short. But their relative 

 extent varies considerably : the small intes- 

 tine being sometimes lengthened, and often 

 presenting a very pecnliar appearance in the 

 shortness of its mesentery and the closeness 

 of its folds. The indistinct ilio-caecal valve is 

 chiefly marked by a change in the diameter of 

 the tube. The large intestine is often sub- 

 divided into distinct portions by one or two 

 transverse valves. 



In the Chelunia the intestine is longer and 

 much more muscular. There is generally 

 an ilio-csecal valve, and often a well-marked 

 caecum. The valve is also present in most 

 of the Sauna. But in the Crocodile the caj- 

 cum is absent. 



Birds. In this class, the stomach is generally 

 complex; being separated into three distinct 

 cavities, which differ greatly in their form, 

 structure, and office. 



The cesophagus, which leads to the first 

 of those cavities, has a length corresponding 

 to that of the neck which it occupies. Its 

 width and dilatability mainly depend on the 

 nature and form of the food. Thus, in some 

 of the birds of prey, or those which swallow 

 large fish entire, it is very lax and dilatable. 

 And in this respect, as well as in the direct con- 

 tinuity of its cavity with that of the stomach, 

 it offers a great similarity to the gullet of 

 the Ophidian reptiles and many fishes. Its 

 mucous membrane is follicular, and folded 

 longitudinally. 



The ingluvics, or crop, is a dilatation of the 

 oesophagus, somewhere about the middle of 

 its length. In some of the smaller Rap- 

 tores it is but small ; in the larger and more 

 voracious it is a considerable enlargement, 

 that affects one side of the tube more than 

 the other ; in the Gallinaceans it is a distinct 

 sac, appended to the canal by a narrower 

 neck ; and, finally, in the Pigeons, it attains 

 its maximum size, and becomes double. Its 

 muscular and mucous membrane are similar 

 to those of the oesophagus. The food which 

 it contains undergoes a kind of insalivation 

 and maceration. And the highly-developed 

 form of crop, which is seen in the Pigeon, 

 pours out a milky fluid during that period 

 of the year in which this bird feeds its young 

 by regurgitation. At this time its mucous 

 membrane also acquires a thicker and more 

 glandular character. 



The proper stomach, or proventricitlus, com- 

 municates with the inferior part of the oeso- 

 phagus, and corresponds, both in structure and 

 function, with the true stomach of the Mam- 

 malia. The glandular tubes which open on its 

 free surface secrete a fluid that possesses all 

 the properties of gastric juice. In the degree of 

 complication these glands differ considerably ; 



varying from simple tubes in the carnivorous 

 birds, to tubes that open between prominences 

 and prolongations, and finally form primary 

 and secondary branches. The shape and size 

 of this organ are subject to great variety in 

 different genera. In those that swallow a 

 large prey, it is wide and straight, like the 

 stomach of the Serpent. In others, it ap- 

 proaches the spherical form, or passes towards 

 the right side to join the gizzard. The com- 

 parative size of these two organs also varies 

 considerably. 



The gizzard is a flattened ovoid of highly 

 muscular texture. It is lined by a dense 

 horny cuticle, and contains sand, gravel, or 

 other hard inorganized matters, which are the 

 passive agents in the trituration of the food. 

 Its size varies greatly. Its apertures both 

 occupy the upper part of the organ, so that 

 its cavity terminates below in a blind ex- 

 tremity. Its walls contain a variable amount 

 of muscle, the arrangement of which is usually 

 that of the radiation of fibres from a central 

 tendon, such as was previously noticed in 

 speaking of the Cephalopoda. Its epithelium 

 is hardest in the granivorous birds. And even 

 in the same individual, it offers an increased 

 density at the precise situations of most 

 pressure. In like manner, Hunter noticed 

 that a thickening, both of cuticle and muscle, 

 was produced by feeding a Sea-gull upon grain. 



The pyloric valve is, as a rule, well marked. 

 In some species there is a small supplementary 

 cavity, which immediately precedes it, and 

 receives the orifice of the gizzard. 



The intestine has a length about midway 

 between that of the Reptilian and Mammalian 

 bowel. But although longer than in either of 

 the preceding classes, it retains considerable 

 simplicity of form. It presents, however, 

 much variety, both in its length and in the 

 number and appearance of its convolutions ; 

 differences which, as usual, are related (though 

 not very closely) to the food and habits of the 

 animal. The duodenum which immediately 

 follows the pylorus has the form of a long 

 loop or fold, the concavity of which includes 

 the pancreas. The small intestine, more or 

 less folded, terminates in a large intestine, the 

 commencement of which receives two caecal 

 tubes, one on each side. These caeca offer 

 remarkable differences in length : varying from 

 papilliform offsets, as in the Solan-goose, to 

 processes three feet long, as in the Grouse. 

 Sometimes only one is present. The short 

 and straight large intestine is continued from 

 the termination of the small intestine, without 

 any distinct valve, to end in a cloaca com- 

 mon to the digestive, urinary, and generative 

 organs. Connected with the small intestine 

 is an appendage, supposed to be a relic of 

 the duct of communication between the 

 yolk bag and intestine of the chick. It is de- 

 void of a muscular tunic, and in some birds 

 equals or exceeds the size of the caeca them- 

 selves., 



Mammalia. The form, length, and arrange- 

 ment of the alimentary canal vary so much 

 in the different orders of Mammalia, that it 



