DIGESTION IN THE MOUTH. 



295 



pharyngeal — the nerves of taste. The centre is in the medulla oblon- 

 gata and the efferent fibre is the chorda tympani, a branch of the seventh 

 — the facial — nerve (Fig. 119). The influence of these nerves on the 

 secretion of saliva is readily proved by experiment. 



The animal on which this experiment is usually performed is the dog. The 

 operation is performed as follows : — A large dog is chloroformed and fastened in 

 Bernard's dog-holder. The hair is # shaved from the lower surface of the jaws and 

 the side of the neck, and an incision made along the lower border of the lower 

 jaw, commencing about its anterior third and extending back to the transverse 

 process of the atlas, dividing the skin and platysma muscle. After clearing 

 away the connective tissue and fat, carefully avoiding the veins, the submaxillary 

 gland comes into view just below the angle of the jaw. It is then seen that the 

 gland lies in the angle formed by the junction of the two veins which go to make 

 up the external jugular vein (Fig. 120), one branch coming from above downward 

 directly behind the gland, and usually receiving a small vein from the gland 

 itself, while the lower branch runs horizontally below the gland, and is formed 

 by the junction of two other branches, one coming from above and the other from 

 below. The horizontal branch also very constantly receives a vein from the 



Fig. 120.— Veins of the Submaxillary Gland of the Dog. (Bernard.) 

 g, submaxillary gland ; j, external jugular vein dividing into two branches ; j ' and j", veins which sur- 

 round the gland ; d, anterior glandular vein ; dt, posterior glandular vein. 



gland. Both branches which go to form the horizontal branch are tied, the one 

 coming from above receiving a double ligature where it comes from the ramus 

 of the jaw, and the otherwhere it joins its fellow, the intermediate portion being 

 removed. After having carefully removed the cellular tissue from the portion of 

 the wound in front of the gland, the thick belly of the digastric muscle comes 

 into view, its fibres running forward from its origin in the temporal bone to be 

 inserted in the middle third of the ramus of the lower jaw immediately in front 

 of the insertion of the masseter, from which muscle it is separated by a slight 

 groove. In front of the digastric, the floor of the wound is formed by the trans- 

 verse fibres of the mylo-hyoid muscle, crossed by the mylo-hyoid nerve, which 

 comes out from under the jaw at the point of insertion of the digastric muscle. 

 The connective tissue is then gradually to be cleared away with a blunt hook 

 from the surface of the digastric muscle and from the groove between it and the 

 masseter muscle, taking care to avoid, as the deeper portion is reached, the facial 

 artery, which passes over the jaw to run between these muscles, and the artery of 

 the gland, which comes from the facial artery and goes in this groove back to. 

 the gland. In the same locality lie also the ducts of the gland and the chorda 

 tympani nerve. The digastric muscle is now to be separated by means of an 

 aneurism needle from the facial artery, avoiding all the adjacent structures, and 

 its muscular arterial branch tied. The muscle is then divided at its anterior third. 



