THE BACK. 417 



seventh nerve. This nerve communicates with the third division of the fifth 

 nerve ; with the facial, sympathetic, and vagus. 



What can you say of the function of the vagus ? 



In animals less highly specialized than man the term pneumogastric 

 expresses practically all that can be said of this nerve, since in its wandering 

 distribution it supplies the organs of voice and respiration with motion and sensa- 

 tion ; the organs of circulation and digestion with motion only. The vagus, 

 then, has to do with the two greatest sources of metabolism : the ingestion of food 

 and air. Man is not all stomach and lung. By synecdoche, however, stomach 

 may represent a complicated digestive apparatus, and lung an equally differ- 

 entiated apparatus, in which the blood throws off CO 2 and loads up with O, 

 according to the law of the diffusion of gases. Now, the student must remember 

 this : The names of the branches of the vagus nerve will be governed and deter- 

 mined by the following conditions : 



1. The arbitrary subdivision of the alimentary canal, to which the vagus is 

 distributed. 



2. The arbitrary subdivisions of the respiratory system, to which the vagus is 

 distributed. 



3. The sympathetic nerve-branches to both the respiratory and digestive 

 systems, since the sympathetic is the nerve of organic life. 



4. The organs, or systems of organs, associated secondarily with the respira- 

 tory and digestive processes. The tongue, for example, is associated with both 

 respiratory and digestive processes, and while this organ has its own specific nerve- 

 supply, upon which its sensation and motion depend, still, the orderly adjustment 

 of internal to external relations could not occur, were there no communication 

 between the nerve-supply of the tongue, which forces food into the pharynx, and 

 the organs that deliver this food to the stomach. In like manner there must be 

 unanimity of action between the lungs, vocal cords, and tongue, for vocalizing and 

 linguistic purposes ; this unanimity must depend on an uninterrupted communi- 

 cation between the nerve-supply of the tongue and the vagus nerve. Observe 

 the difference in facial expression of the two men, the one of whom is in the 

 nausea period of a malarial attack, the other of whom is enjoying an after-dinner 

 cigar. The facial nerve supplies the muscles of expression, and by its communi- 

 cation with the vagus both comfort and distress in the stomach are facially 

 expressed. 



PHYSIOLOGICAL REASONS FOR THE NUMEROUS COMMUNICATIONS OF THE VAGUS 



NERVE. 



1 . With the spinal accessory nerve. The spinal part of the spinal accessory 

 nerve is distributed to the sterno-cleido-mastoid andtrapezius muscles, and assists 

 in forming the cervical plexus. It will be remembered that the mission of the 

 cervical plexus is to supply the diaphragm and certain muscles of forced respira- 

 tion, as well as the depressor muscles of the hyoid bone. (Page 73.) 'Now, the 

 accessory part of the spinal accessory nerve supplies the muscles of the soft 

 palate. Thus, by the communication between the vagus and the spinal acces- 

 sory, harmony is established between the muscles that rhythmically enlarge and 

 reduce the size of the thorax, and the lungs that occupy this thorax, and by 

 their rhythmic movements carry on vocal and respiratory processes. 



2. I Vith the petrous ganglion of the glosso-pharyngcal or ninth cranial nerve. 

 This nerve has a distribution as follows : (i) To the tongue ; (2) to the pharynx 

 via the pharyngeal plexus ; (3) to the middle ear, Eustachian tube, and mastoid 

 cells via the tympanic plexus ; (4) to the parotid gland via its small superficial 

 petrosal branch and the otic ganglion ; (5) to the facial nerve ; (6) to the sympa- 



