522 SPECIAL PHYSIOLOGY. 



beneath the gland. The saliva from the sublingual glands flows into 

 the mouth beneath the tip and sides of the tongue. 



The mechanical flow of the saliva into the mouth is aided by the 

 contraction of the muscles of the tongue and jaw engaged in mastica- 

 tion ; on opening the mouth before a looking-glass, and then turning 

 up, and stiffening, the tongue, the saliva is sometimes seen to be ejected 

 a considerable distance, from the orifices of the Whartonian ducts. 



The salivary glands all receive branches from the sympathetic ner- 

 vous system ; the parotid glands are likewise supplied by the fifth pair 

 (its auriculo-temporal branch) ; whilst the sublingual and submaxillary 

 glands receive nervous filaments from the chorda-tympani branches of 

 the facial nerves. The saliva flows intermittently ; and its secre- 

 tion is excited through the nervous system, by the agency of which, 

 the quantity of this and other secretions, is chiefly regulated (p. 

 264). Thus, the presence of food, especially of dry food, in the 

 mouth, and even the introduction of food into the stomach through a 

 gastric fistula, stimulates the flow of saliva ; salt, vinegar, pepper, and 

 other condiments, and particularly tobacco, and the root of the pelli- 

 tory of Spain, have a still more powerful effect ; these furnish exam- 

 ples of reflex stimulation of the salivary secretion. The afferent 

 nerves concerned, are the gustatory branches of the fifth pair, and the 

 glosso-pharyngeal nerves ; the efferent nerve-fibres are contained in 

 the chorda-tympani branches of the facial nerves, or in the auriculo- 

 temporal branches of the fifth pair. The nervous centres are the sub- 

 maxillary ganglia, and the cerebro-spinal axis. Besides this, the 

 saliva is excited to flow by ideational or other mental stimuli, such as 

 the sigbt of food, or even the thought of it. The act of speaking, 

 and also that of vomiting, are preceded by a flow of saliva. Fear 

 diminishes or arrests it. Irritation of the fourth ventricle, and the 

 presence of certain substances in the blood, especially of mercury, 

 likewise increase the flow of this secretion. The effect of mercurial- 

 ization in exciting a flow of saliva is specific. 



The mode in which the nervous system influences the secretion of 

 saliva, has been elucidated by the interesting experiments of M. Ber- 

 nard. When the sublingual and submaxillary glands, exposed in an 

 animal, are at rest, little or no saliva being formed, the veins are seen 

 to contain a moderate quantity of dark blood. On now stimulating 

 the glands, in a reflex manner, by the application of vinegar to the 

 tongue, the arteries supplying them dilate, the flow of blood through 

 these vessels becomes quicker, even the veins pulsate, the venous blood 

 is of a bright red color, and there occurs a copious flow of watery 

 saliva. The afferent nerves concerned in this reflex act, are obviously 

 branches of the gustatory and glosso-pharyngeal nerves ; the efferent 

 fibres are contained in the chorda tympani; for if either this or the 

 facial, from which it is derived, be cut, the active phenomena above 

 described, all gradually cease, but they are again excited by irritation 

 of the distal ends of the divided nerves. If the facial nerve be drawn 

 out from the cranial cavity, irritation of the glosso-pharyngeal no 

 longer increases the flow of saliva. The efferent nerves of the parotid 

 glands are said, by Eckhard, to proceed, not from the chorda tympani 



