106 PSYCHOBIOLOGY 



themselves. The activity of a gland can be altered by nerve currents 

 affecting the cells directly and by changes in the volume of the blood sup- 

 ply produced by contraction or enlargement. of the blood vessels as well as 

 by the influence of substances (such as C0 2 or the secretions of the ductless 

 glands) brought to the gland cells in the blood. 



THE GENERAL STRUCTURE OF THE ALIMENTARY CANAL. 



The consecutive gross divisions of the alimentary canal are the mouth 

 cavity, the pharynx or throat, the esophagus or gullet, the stomach, 

 the small intestine, and the large intestine. The stomach connects 

 with the gullet through the esophageal orifice and with the small intes- 

 tine through the pylorus. The small intestine is divided into duodenum, 

 jejunum and ileum, the first being the upper ten or eleven inches of the in- 

 testine and distinguished from the remainder both structurally and func- 

 tionally. The large intestine is divided into ccecum, colon, and rectum. 



The entire alimentary canal is lined with mucous membrane, con- 

 sisting of a surface layer of stratified epithelium resting on a layer of con- 

 nective tissue called the stroma or tunica propria, with sometimes a base- 

 ment membrane separating the two. Beneath the mucous membrane are 

 muscular and connective-tissue structures which, in the gullet, stomach 

 and intestines, take on the form of definite coats. 



The four coats of the gullet, stomach and intestines are therefore : 



1. The mucous membrane. The lowest stratum of the stroma (in the 

 organs mentioned: not in the mouth and pharynx) is a sheet of smooth 

 muscle fibres. 



2. The submucosa. This is a loosely attached layer of areolar con- 

 nective tissue. 



3. The muscular coat. In the upper part of the gullet this is composed 

 of striated muscle; in the middle portion, of both smooth and striated 

 fibers; in the lower portion of the gullet and throughout the stomach, 

 small intestines, caecum and colon, there are smooth fibers only. In the 

 gullet and intestines the muscle fibers are arranged in two layers, an inner 

 circular layer and an outer longitudinal layer. In the stomach there are 

 three layers. 



4. Surrounding the muscular coat of the upper part of the gullet is a 

 coat of areolar connective tissue loosely joining it to the adjacent struc- 

 tures. The lower part of the gullet, stomach and intestines have a serous 

 coat of smooth connective tissue, the peritoneum, which is continuous 

 with that lining the abdominal cavity. From the serous coat of the stomach 

 folds of peritoneum called omenta (singular omentum) pass to the large 

 intestine, to the liver, and to the spleen, connecting the stomach with these 



