SALIVA AND SALIVARY DIGESTION 399 



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 acidj 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 glands is well known, and the anato- 

 mical relations are such as to make these glands favorable for studying the 

 nervous 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 

 submaxillary gland. Isolate and cut the digastric muscle, also the mylo- 

 hyoid, using pains not to injure the duct of the gland or its arteries. When 

 the muscles are laid back', the artery and accompanying sympathetic nerve 

 branches, the hypoglossal and the lingual nerves, the submaxillary duct and 

 the submaxillary gland, will all be exposed. Isolate and introduce a very 

 fine glass cannula into the submaxillary duct tying it firmly in place. A small 

 nerve filament branches from the lingual nerve and runs to the hilus of the 

 gland, the chorda tympani. Carefully expose the chorda, place a silk liga- 

 ture under it for convenience in handling. Also expose the sympathetic 

 filaments with the artery. 



Stimulate the chorda tympani with a mild induction current for a few 

 1 minutes at a time at intervals, and note that the secretion which is absent or 

 forming slowly before stimulation now gathers quickly and leaves the end 

 of the cannula in a series of drops. Collect the saliva in a small beaker. One 

 can measure the rate of flow by collecting the saliva in a small graduated 

 cylinder or, by changing the beaker every ten minutes, making a record of 

 the quantity of secretion formed. Stimulate the sympathetic fibers, cutting 

 the hypoglossal nerve if necessary, and note that the secretion is very slightly 



