28 



Basic Structure of Vertebrates 



MUSCULAR IS MUCOSAE 





t LONGITUDINAL 

 ®7W|| { MUSCLES 



ADVENTITIA 



VAGUS NERVE 



Fig. 20. The esophagus as seen in cross section. (Right) A section of the entire 

 esophagus. (Left) A small portion much enlarged. The ldyers of tissue characteristic 

 of the entire alimentary canal are found in the esophagus. (After Braus. Courtesy, 

 Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



rugae on the wall of the stomach (Fig. 21) and the transversely di- 

 rected plicae which may occur in both the "small" and the "large" 

 intestine (Fig. 23). The internal surface of the more anterior region of 

 the intestine is usually thickly beset with delicate, long, cylindric pro- 

 jections, the villi, which give the surface a velvety appearance and 

 texture. Each villus consists of digestive epithelium enclosing a core of 

 connective tissue filled with blood-capillaries and small lymph-vessels 

 (Fig. 22). It is thus equipped for absorbing the digested food which 

 bathes its surface. In cyclostome eels, sharklike fishes, and a few other 

 fishes, an inwardly projecting fold of the intestinal lining extends 

 lengthwise of the tube and winds spirally about it, in some cases mak- 

 ing several or many spiral turns in its course from one end to the other 

 (Fig. 23). This spiral valve greatly increases the functional surface of 

 an otherwise short intestine. When the width of the spiral fold exceeds 

 the radius of the intestine, as it commonly does, the fold necessarily 

 becomes coiled upon itself in the axis of the intestine, thus occluding 



