DIGESTION IN THE MOUTH. 299 



before it passes out of the st} T lo-mastoid foramen, and then, arching 

 upward and forward, enters the middle ear, which it traverses from 

 behind forward, lying within the thickness of the membrana tympani. 

 Here for a space of six or eight millimeters the nerve is comparatively 

 isolated, lying between the handle of the malleus and the vertical process 

 of the incus. It then passes toward the Glaserian fissure, and leaves 

 the skull in the neighborhood of the spine of the sphenoid bone to join 

 the lingual nejve. It has already been stated that stimulation of the 

 central end of the lingual nerve calls forth a secretion of submaxillary 

 saliva. If, however, the chorda tympani nerve be previously divided, 

 stimulation of the lingual is without effect. 



The simplest method of dividing the chorda tympani nerve is to cut it where it 

 crosses the tympanum. This may be accomplished by introducing a small sickle- 

 shaped knife into the external auditory canal, the animal being profoundly chloro- 

 formed, keeping the cutting edge upward, and passing the back of the blade 

 downward and forward along the inferior wall of the meatus until the tympanum 

 is reached. Pushing the blade through the tympanum, the knife is inserted in 

 the middle ear, and on depressing the handle of the knife in this position the nerve 

 is divided. 



The fact that the chorda tympani constitutes the efferent nerve in 

 this reflex circle is not only proved by the experiment just alluded to, 

 where its division prevents the flow of saliva after stimulation of the 

 lingual, but ma} T be positively demonstrated by its stimulation. If the 

 chorda tympani nerve is directly stimulated with a weak induced electrical 

 current just after it leaves the lingual trunk, in a few seconds the saliva 

 begins to flow from the cannula, and runs in quite a stream. 



It has thus been shown that the secretion of submaxillary saliva is 

 a reflex nerve mechanism; that the sense of taste is the normal stimulus, 

 and that this stimulus reaches the brain through the fibres of the lingual 

 and glosso-pharyngeal nerves, and is transmitted to the gland from the 

 medulla through the fibres of the chorda tympani nerve. We have now 

 to study the mechanism by which saliva is separated by the gland from 

 the blood and the influence of the various nerves and different conditions 

 of the circulation on this process. 



If the submaxillary gland is exposed as described above, and the 

 chorda tympani nerve stimulated, not only is there a copious secretion of 

 saliva, but the appearance of the gland itself undergoes great change. 

 If examined before the nerve is stimulated, the gland will usually appear 

 pale. A, few arborescent vessels will be seen upon its surface, and the 

 blood which leaves the gland is dark, and the vein small. When, how- 

 ever, active secretion is produced through stimulation of the chorda 

 tympani nerve the surface of the gland becomes rosy red. Numerous 

 branching vessels are seen. The blood that flows from the gland is 

 almost arterial in hue, is much larger in quantity, and the veins are seen 



