468 



THE RESPIRATORY MECHANISM. 



glossal branches (first, second, and third cervical), and two 

 spinal accessory branches (second and fourth cervical). 



Summarized, the innervation of the muscles of respiration 

 appears to us to be as follows: 



1. The nervous supply of the respiratory muscles is composed 

 (1) of divisions of the general motor system, and (2) of divisions 

 of the vagal system. 



2. The divisions of the general motor system are (1) the 

 phrenic (the internal respiratory nerve of Bell), distributed to the 

 diaphragm; (2) the intercostals, distributed to the parietes of the 

 thorax and abdomen; and (3) the posterior thoracic (the external 

 respiratory nerve of Bell), distributed to the serrati magni. 



3. The divisions of the vagal system are (1) the hypoglossal, 

 distributed to the diaphragm (conjoined to the phrenic) and to the 

 intercostal and external respiratory muscles (instead of sympa- 

 thetic) ; (2) the inferior laryngeal, distributed to the muscles of 

 the larynx (except the cricothyroid) ; and (3) the superior laryn- 

 geal, distributed to the cricothyroid muscle. 



4. The divisions of the general motor system maintain tonic 

 vascular contraction in, and nutrition of, the respiratory muscles, 

 while the divisions of the vagal system incite and govern their 

 functional and co-ordinative activity. 



5. The mechanical energy of the respiratory muscles is the 

 result, as in all muscular tissues, of a chemical action of the 

 oxidizing substance of the Hood-plasma upon the myosinogen of 

 the muscle-cells. 



THE NERVO- VASCULAR MECHANISM OF THE LUNGS. 



The pulmonary circulation as regards general distribution 

 is succinctly portrayed in the following description by Miller, 25 

 as given by Bohm and von Davidoff 26 : "The pulmonary artery 

 follows closely the bronchi through their entire length. An 

 arterial branch enters each lobule of the lung at its apex, in 

 close proximity to the bronchus. After entering the lobule 

 the artery divides quite abruptly, a branch going to each in- 

 fundibulum; from these branches the small arterioles arise 

 which supply the alveoli of the lung. 'On reaching the air-sac 



2 Miller: Journal of Morphology, vol. viii, p. 165, 1893. 

 28 Bohm and von Davidoff: Loc. cit. 



