34 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



Digestive Tract. — The alimentary tract proper begins be- 

 hind the pharyngeal region and extends to the vent. In Pe- 

 -tromyzon, of the cyclostomes, a growth from the floor of the 

 hinder portion of the pharyngeal region extends forward above 

 ' the gill slits, so that separate respiratory and digestive tubes 

 occur in this region. In other vertebrates there is no such 

 separation. In the cyclostomes the alimentary tract shows but 

 slight differentiation into regions, the point of entrance of the 

 liver duct serving to divide it into pre- and post-hepatic por- 

 tions. In the latter division a slight fold of the internal sur- 

 face forms a rudimentary spiral valve recalling that to be 

 described below in elasmobranchs and ganoids. In the holo^ 

 cephali, some teleosts, and the lower urodeles there is scarcely 

 more differentiation of the digestive canal. 



In all other forms the digestive canal is more or less clearly 

 divided into regions. Thus we find the pre-hepatic portion dif- 

 ferentiated into an anterior slender tube, the gullet or oesophagus, 

 and a posterior widened portion with glandular walls, the stom- 

 ach. The oesophagus calls for few remarks. Its length is cor- 

 related with that of the neck, and onl\" in certain birds is any 

 marked differentiation in its walls to be seen. Here it becomes 

 widened near its middle into a glandular sac of variable form, 

 the ingluvies or crop, which serves as a reservoir of food, and 

 in the pigeons furnishes a food for the ^-oung. 



The stomach, on the other hand, presents numerous modifi- 

 cations. Behind it is usually sharply marked off from the rest 

 of the alimentary tract by an internal fold, and by a well-devel- 

 oped sphincter muscle in its walls. This forms the pylorus. 

 The opposite end of the stomach is the cardiac region, so called 

 since in man it lies nearest the heart. The stomach may be 

 parallel with the axis of the body, but usually, as in most fishes, 

 it is loop-like, or comes to lie more or less at right angles to that 

 axis, conditions brought about" by a lengthening of the tract 

 more rapidly than the body increases in length. Correlated 

 with the absence of teeth, the stomach of the bird acquires a 



great development, and becomes divided into two chambers, an 



anterior glandular portion, the proventriculus, and a posterior 

 muscular portion, commonly known as the gizzard. When most 



