F. ORGANS OF NUTRITION. 



ALIMENTAIiY CANAL AND ITS APPENDAGES. 



THE alimentary canal (tractus intestinalis) consists of a tube 

 A\hich begins at the aperture of the mouth, passes through the 

 body cavity (ccelome), and ends at the anus. Its walls consist 

 essentially of three layers; an inner epithelial, a middle con- 

 nective-tissue, and an outer muscular layer. The first, 

 which corresponds to the hypoblast of the embryo, forms the lining 

 of the canal (Fig. 8, Ej)}, and gives rise to numerous glandular 

 structures which have a secretory as well as a resorptive nature; 

 the second (Sub-in), consisting of connective and adenoid tissue, 

 serves chiefly to conduct the blood and lymph vessels ; while the 

 third (JAr) which, together with the second, corresponds to the 

 splanchnic layer of mesoblast of the embryo, is, as a rule, divided 

 into two layers, and consists of smooth muscular elements, the inner 

 being constituted by circular fibres, and the outer by longitudinal 

 ones. These serve for the contraction of the wall of the gut, and 

 thus fulfil the double function of bringing its nutritive contents 



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into the closest possible relation with the whole epithelial surface, 

 and at the same time of removing from the body the substances 

 which have not been absorbed. A fourth accessory serous coat, 

 enclosing the gut externally in the region of the ccelome, must be 

 added to these three layers. It is covered on its free surface by 

 pavement epithelium, and is reflected round the entire body-cavity, 

 converting the latter into a large lymph-sinus. Its abdominal 

 portion is spoken of as the peritoneum, and its thoracic portion 

 as the pleura, the heart being invested }>y a special serous mem- 

 brane, the. pericardium. In the cranial and cervical portions of 

 the alimentary tract the serosa is not developed. 



A parietal layer, lining the body-cavity, and a visceral 

 layer reflected over the viscera, ran then be distinguished in 

 the peritoneum (Fig. <S, !'<,-, /'</'). The region where one passes 

 into the other, which is thus primitively double, is called 

 the mesentery ' .)/,s), and this serves not only to support the 

 alimentary canal from the dorsal body-wall, but also to conduct 



