ALIMENTATION 



95 



and intestine there is another muscular layer, the muscularis 

 mucosae, which is composed of an inner circular and an outer 

 longitudinal layer. Both are very thin. The inner surface of 

 the esophagus is thrown into longitudinal folds. The mucosa 

 consists of columnar goblet cells 

 among which ciliated cells are 

 found similar to those lining the 

 oral cavity. The frog's esopha- 

 gus is provided with tubular 

 glands that secrete pepsin, which, 

 however, does not function until 

 it reaches the stomach. 



The muscle tissue found in the 

 walls of the vertebrate alimen- 

 tary tract and also in the walls of 

 blood vessels consists of mono- 

 nucleated, spindle-shaped cells 

 (Fig. 62), without the striations 

 found in skeletal and cardiac 

 muscle. It is known as plain or 

 nonstriated muscle and, as 

 already noted, is involuntary in 



its action. Fig. 61. — Diagrammatic cross sec- 



The innervation of the esopha- $ on of . half of 1 lower **>&**** of 



c nana pipiens. 1, serous membrane or 



gUS as Well as Of the remainder visceral peritoneum; 2a, longitudinal 



of the alimentary tract is from muscle; f ' drcular T 8C !, e; 3 ' sub_ 



J mucosa; 4, mucosa; g, glands. 



branches of the sympathetic nerv- 

 ous system and the vagus nerve, that penetrate the tunica 

 muscularis to form the myenteric plexus between the two layers 

 of muscle and the submucosal plexus in the submucosa. 



Fig. 62. — Plain muscle cells of the muscularis layer of intestine of Rana pipiens. 



The crop or ingluvies of birds is a differentiated, saclike 

 expansion on one side of the esophagus serving as a receptale for 

 food. Sometimes its wall forms a secretion that moistens the 

 food or starts its digestion. In pigeons the crop, consisting of a 



