260 HISTORY OF THE HUMAN BODY 



formed by stomato- and proctodaeum, and therefore ectoder- 

 mic, is extremely large, and the mesodseum, though volumi- 

 nous, is employed mainly in the formation of laterally placed 

 digestive glands; but in the vertebrate the canal is mainly 

 mesodseal, and therefore endodermic, the ectodermic oral and 

 anal contributions being much restricted. 



With the exception of a small number of auxiliary organs 

 like the jaws, teeth, and tongue, the entire digestive system is 

 derived from this simple tube, and all the parts which appear 

 in even the most complicated cases develop from this by 

 means of such mechanical principles as increase in length, local 

 enlargement, foldings, outpushings, and inpushings, in short, 

 such principles as are employed in developmental history every- 

 where. More than this, from the anterior portion of this canal 

 there develop the two principal respiratory systems of verte- 

 brates, the branchial or gill system for aquatic breathing, and 

 the pulmonary or lung system for air. This close association 

 between digestive and respiratory systems is essentially a ver- 

 tebrate characteristic and is hardly known among other ani- 

 mals, save in the cases of Amphioxus, the tunicates and the 

 Enteropneusta, which in other respects also show their close 

 affinity to the Vertebrata. [See Chap. XII. ] Although so 

 closely related anatomically, the digestive and respiratory sys- 

 tems are best disassociated in treatment and. will be considered 

 separately as far as possible. 



Although essentially and in its origin an endodermic organ, 

 the alimentary canal always becomes reinforced by other tis- 

 sues which form layers outside of the primary endodermic one. 

 Including the latter the layers are usually considered four ii 

 number, named in order, beginning with the inner one : mi 

 cosa, submucosa, musculosa, and serosa. The mucosa is tl 

 primary agent in digestion and develops glands for the pro- 

 duction of the various necessary digestive juices; it ali 

 contains a thin layer of involuntary muscular fibers and 

 permeated with blood and lymphatic vessels for absorbing ai 

 carrying away the nutriment when in a proper condition fc 

 assimilation. The submucosa is a thin layer of connective 



