GENERA L ORNITHOL OGY 



Being always longer than the body, the tube is necessarily coiled 

 away in certain places ; this folding taking place chiefly in the 

 intestinal part of the tract. Modifications of structure make recog- 

 nisable parts, as the mouth, gullet, crop, stomach, gizzard, intestine, 

 cloaca, anus. Annex organs are the salivary glands, the liver, and 

 the pancreas, all of which pour their secretions into the canal. This 

 tube also receives the terminations of other systems of organs : tlie 

 auditory organ of special sense ; the respiratory system, which 

 is at first a mere bud or offset from the digestive ; the urinary 

 and the generative, which, though originally distinct, primitively 

 and pei'manently open into the lower bowel. The intestine is also 

 continuous with the cavity of the umbilical vesicle of the embryo, 

 a })rimitive structure which disappears as the chick matures ; and 

 Avitli that of the allantois, another embryotic organ which begins by 

 budding from the intestinal cavity. Its connection with the system 

 of blood-vessels is direct through the lacteals and thoracic ducts 

 (p. 295). Its operations are automatic and spontaneous, of the 

 " reflex " order ; that is, excited by the presence of food, — having 

 Avork to do making it work, so to speak. Its innervation is chiefly 

 by the pneumogastric and sympathetic nerves ; and digestion is the 

 most purely vegetative function, dealing with the raw materials of 

 nutrition and consequently of the growth and repair of the whole 

 body. The active factors in this transaction are several species or 

 varieties of small creatures, called Entcrmna'ha' ; they are all derived 

 by descent with modification from the hypoblastic cells of the early 

 embryo. Those of the canal itself form all the mucous epithelium 

 of that structure, Avith its various secretory crypts, follicles, and villi ; 

 similar creatures, perhaps of different genera, form the lining of the 

 salivary, hepatic, and pancreatic glands. Blood-vessels in intimate 

 connection with the digestive organs form that special venous 

 arrangement by which the blood coming from that part of the 

 intestinal tract where chyle is made is collected in a portal system 

 and sent through the liver, — in the embryo a sort of " great dismal 

 swamp " which interrupts the ordinary current. The tube within the 

 tube is fixed not only at its ends, but by various membranous con- 

 nections, among them the iiiesenterics. We will notice the several 

 departments of the alimentary canal and its annexes ; reference 

 should be made to Fig. 101, where most parts of the digestive system 

 are shown. 



The Mouth and Tongue. — The most anterior of the special 

 cavities into which the tube is divided, and the " manual " organ 

 it contains. The mouth in general corresponds to the shape 

 of the jaws, already sufficiently noted. The anterior part is 

 much hardened, like the beak ; in fact, this hardness of the buccal 

 cavity, and the absence or very slight distinction of a " soft palate," 



