1528 



HUMAN ANATOMY. 



anal opening. Associated with the mid-gut are two important glands, the liver and 

 the pancreas. Greater complexity in the character of the food and in the manner of 

 securing it necessitates increased specialization in the first segment of the digestive 

 tube ; hence the addition of accessory organs, as the lips, oral glands, tongue, and 

 teeth, the latter often serving as prehensile as well as masticatory organs. 



Reference to the early relations of the embryo to the vitelline sac (page 32) 

 recalls the important fact that the greater part of the gut-tract is formed by the con- 

 striction and separation of a portion of the yolk-sac by the approximation and closure 

 of two ventral folds, the splanchnopleura. Since the latter consists of two layers, the 

 entoblast and the visceral lamina of the mesoblast, the tube resulting from the union 

 of the splanchnopleuric folds possesses a lining directly derived from the inner germ- 

 layer, supplemented externally by mesoblast. The latter contributes the connective 

 tissue, muscular and vascular constituents of the digestive tube, while the epithelium 

 and the associated glandular elements are the products of the entoblast. 



MUCOUS MEMBRANES. 



The apertures of the digestive, respiratory, and genito-urinary tracts mark loca- 

 tions at which the integument becomes continuous with the walls of cavities and 

 passages communicating with the exterior. The linings of such spaces constitute 



Fig. 1282. 



Epithelium 



^_Papilla of tunica 

 propria occu- 

 pied by blood- 

 vessels 



Connective- 



8^,€i«i.^:;s:r>w^^lS4 





^^J^-^- 



^SL 





tissue stroma 



-r^-Z'^^ 



Section of oral mucous membrane. X 350. 



mucous membranes. The latter, however, not only form the free Surface of the 

 chief tracts, but also that of the ducts and tubes continued into the glands which are 

 developed as outgrowths from the mucous membranes. 



Temporarily in the higher types and permanently in such of the lower animals 

 as possess a common cloacal space, all the mucous membranes of the body are con- 

 tinuous. After acquiring the definitive arrangement whereby the uro-genital tract 

 becomes separated from the digestive tube, these membranes in man and mammals 

 (except monotremata) form two great tracts, the gastro-pulmonary and the genito- 

 urinary. 



The free surfaces of the mucous membranes are kept continually moist by a 

 viscid, somewhat tenacious secretion, the mucus, derived from the glands ; they are 

 thus protected from the drying and irritating influences of the air, foreign substances, 

 and secreted or excreted matters with which they are brought into contact. 



Structure. — Every mucous membrane comprises two distinct parts : the epi- 

 thelium, which forms the immediate free surface and furnishes protection for the more 

 delicate tissues beneath ; and the tiinica propria, a connective-tissue layer which 

 constitutes the stroma and gives place and support to the terminal branches of the 



