SALIVA AND SALIVARY DIGESTION 377 



movements. The intestine carries out peristalses when isolated from the 

 body so that the central connections do not originate, but are only regulative. 

 The intestinal movements are essentially automatic, depending on the rhyth- 

 mic property of the muscle itself, but co-ordinated by the complex local 

 nervous mechanism. 



The innervation of the large intestine is also double in character and 

 the relations are doubtless the same as in the small intestine. 



Defecation. The emptying of the rectum is essentially an involun- 

 tary act which has acquired a certain amount of voluntary regulation. The 

 act is accomplished wholly reflexly in dogs with isolated lumbar cord, in fact 

 has been observed when the lumbar spinal cord was removed. In the latter 

 case defecation occurs by automatic peristalsis of the rectum, while in the 

 former cases reflexes through the lumbar cord carry out the act. The 

 stimulus of the feces against the rectum and the internal sphincter initiate 

 the movement. 



Normally in man the rectal stimulus gives rise to the consciousness of 

 the desire to defecate and to the initiation of efferent nerve impulses that may 

 increase the contraction of the external sphincter and inhibit the act tempo- 

 rarily. During defecation, however, the voluntary effort leads to relaxation 

 of the external sphincter, and the normal peristalsis of the rectum is sup- 

 ported by contractions of the abdominal musculature so as greatly to increase 

 the abdominal pressure, thus aiding the involuntary reflex. 



LABORATORY EXPERIMENTS IN DIGESTION. 

 I. SALIVA AND SALIVARY DIGESTION. 



1. Reflex Salivary Secretion. Saliva, which is the mixed secretion 

 of the salivary and buccal glands, is produced more or less intermittently. 

 Examine, taste, or smell appetizing food, for example, an apple, the salivary 

 glands begin to discharge secretion which is poured into the mouth more 

 rapidly than under ordinary conditions. This increased activity is a reflex 

 secretion. It is brought about by the stimulation of sensory structures which 

 lead to afferent nerve impulses reacting on nerve centers in the medulla to 

 cause secretory nerve impulses to the glands. The stimulating effect of food 

 in the mouth causes the most rapid reflex secretion, which may last through 

 several minutes or even hours. Especially stimulating substances are, 

 beside food, such substances as tartaric acid, lemon juice, ether, alcohol, etc., 

 in fact, anything that produces strong local irritation will lead to reflex 

 secretion. 



2. The Secretory Nerves of the Salivary Glands of the Dog. The 

 nervous mechanism for the salivary gland is well known, and the anatomical 

 relations are such as to make this gland a favorable one for studying the nerv- 

 ous mechanism of glands in general. 



