26 



Basic Structure of Vertebrates 



Branchial 



Spiracle 



clefts 



Vertebral column 

 Spinal cord 



Spiracle 



Heart Pectoral girdle 



Mouth-cavity 



Pharynx 



Esophagus 



vertebral column 



Neural arch 



Mouth-cavity 



Pharynx 



Esophagus 



Fig. 18. Divisions of digestive tube. (A) Shark. (B) Beptile. Begions sectioned 

 are cut in sagittal plane, except the nasal sac which, being one of a pair of lateral 

 organs, is shown in parasagittal section. 



Throughout the greater portion of the length of the digestive tube, 

 the muscular part of its wall consists of an external layer of longitudi- 

 nal muscle fibers and an internal layer of circular fibers encircling 

 the tube (Fig. 20). The fibers are ordinarily of the simpler "smooth" 

 or nonstriated sort, but in the more anterior part of the esophagus the 

 layers may consist of the more complex striated fibers (see p. 312). 

 Sphincters, such as the pylorus, are thickened rings of the circular 

 layer. 



The extent of internal surface available for the secretory and ab- 

 sorptive activities of the tube is increased in various ways. The ab- 

 dominal part of the tube is usually much elongated, even to the extent 



