SALIVA AND SALIVARY DIGESTION 351 



mic property of the muscle itself but coordinated 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 a rd 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 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 supported 

 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. Ex- 

 amine, 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 any- 

 thing 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. 



Anesthetize a dog and bind it to a suitable holder. Expose the nerves to 

 the submaxillary gland as follows: cut through the skin of the lower jaw along 

 the inner border for about 3 inches. Isolate and double ligate the jugular 

 vein and any other veins in the field except the ones coming from the sub- 



