298 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



downward and forward, about parallel in direction with the hypoglossal. On 

 drawing the parts toward the middle line, the two salivary ducts are seen passing 

 along close together, immediately below the ramus of the jaw, the submaxillary 

 duct lying nearer the bone and being a little the larger. On tracmg back the 

 lingual nerve to where it passes from under the jaw, it will be seen that a delicate 

 nerve-filament here leaves the lingual and curves backward along the ducts to 

 enter the hylum of the gland. This is the chorda tympam. Immediately after 

 the chorda leaves the lingual there is sometimes seen a small ganglionic enlarge- 

 ment known as the submaxillary ganglion, and as the chorda tympam enters the 

 hvlum it forms a slight ganglionic plexus with the sympathetic. The nerve- and 

 blood-supply of the submaxillary gland ot the dog are further shown in Fig. 122. 

 Each of these nerves which it is desired to study should be carefully isolated and 

 surrounded with a thread, and a cannula should be inserted into the submaxillary 

 duct To facilitate this, the duct should be freed slightly from the connective 

 tissue, and closed with a clip or ligature ; as the gland is passive, the chorda should 

 be stimulated with a very weak electric current for a few seconds, so as to distend 

 the duct with saliva, and a small slip of wood passed under it to act as a support. 

 If the duct is then seized with a pair of fine forceps and snipped with a pair of 

 sharp-pointed scissors, a cannula may be readily inserted. 



The above is the mode of operation employed by Bernard, and permits of 

 the performance of all the more important experiments on the physiology of the 

 secretion of the submaxillary gland. "Where it is simply desired to demonstrate 

 the secretory action of the chorda tympani nerve, the operation may be greatly 

 simplified by simply cutting directly down on to the mylo-hyoid muscle, dividing 

 its fibres transversely, and exposing the ducts and chorda tympani nerve by 

 turning the parts back toward the ramus of the jaw In the sheep, the operation 

 may be performed in the same manner, the duct originating in the union of a 

 number of roots. In the rabbit the operation is much more difficult, from the 

 extreme fineness of the duct and the fact that it is surrounded by the tissue of the 

 sublingual gland. 



After having performed the operation as detailed above, the first 



point which should be demonstrated is the fact that the secretion of 



saliva is a reflex action, and that the reflex circle is as stated above. If 



a few drops of vinegar are placed upon the tongue of a dog provided 



with a submaxillary fistula, almost immediately a profuse secretion of 



saliva will set in, and the fluid will run from the mouth of the tube. If 



the trunk of the lingual nerve is divided near its entrance to the mouth, 



and then vinegar or acetic acid placed on the animal's tongue, no 



secretion will result, unless the stimulating fluid reaches the back of 



the mouth, where it may come into contact with the terminal fibres of 



the glosso-pharyngeal nerve. If the central end of the divided lingual 



nerve is stimulated with a weak electrical current, a profuse secretion 



of saliva will be set up. Therefore the lingual nerve, and, to a certain 



extent, the glosso-pharyngeal, constitute the afferent path by which the 



sensory impressions necessary for the reflex action of saliva reach 



the brain. The nerves of taste are, therefore, the afferent nerves for 



the secretion of saliva. The nerve centre lies in the medulla oblongata, 



and there probably exclusively, although Bernard thought that he had 



shown that, under certain circumstances, the submaxillary ganglion 



might act as a reflex centre for this process. The efferent nerve is the 



chorda tympani. This nerve is a delicate filament which leaves the trunk 



of the facial nerve in the Fallopian canal about four or five millimeters 



