THE GREAT SPINAL ORGANIC NERVE. 
263 
ceeds down the neck, it is joined by other fibrils from both sur- 
faces, even as low as the sixth cervical nerve ; and then it pursues 
it solitary course until it reaches the diaphragm, into the sub- 
stance of which it plunges, and is lost. I need not tell you that 
it is the muscle mainly concerned in respiration ; that in its na- 
tural or relaxed state it bellies into the thorax ; that, stimulated 
by the influence of this nerve, it contracts — it is flattened — and 
the cavity of the thorax is enlarged, and air rushes in through 
the nostrils and the mouth, and the act of inspiration is performed. 
The nervous influence being withdrawn, the muscle relaxes, and 
once more bellies into the thorax, and the cavity is diminished, 
and the air is expelled, and the act of expiration takes place. 
There are other agents at work, but this is the function of the 
diaphragm. If the spinal cord is divided above the origin of the 
phrenic nerve, respiration is immediately impeded, and soon 
ceases, for all the nerves concerned in the dilatation and contrac- 
tion of the chest are cut off. It is an organic nerve, for it is at work 
by day and by night, and whether we are conscious or unconscious 
of its action. It is, to a certain extent, altogether independent 
of the will. The diaphragm, however, can occasionally become 
a voluntary muscle. We can increase or for awhile suspend its 
action when we please. We can do so indirectly by calling in 
the aid of other muscles to accelerate or give force to its action; 
or to oppose a resistance to it which it has not sufficient power to 
overcome : and recent and deeper observation has induced me to 
suspect that it may within certain limits become a muscle of volun- 
tary power. We are conscious of a certain control over its action. 
I can understand this when I contemplate the structure of this 
nerve. I see it springing from the same line with those of sensa- 
tion and motion ; I see it possessing the same characters : I have the 
numerous minute fibrils of the motor nerve, and I have the gan- 
glion of the sensitive one ; at the same time I have every character 
of a true organic nerve. It is at work whether I am thinking 
of it or no ; and though I can to a considerable degree interfere 
with its proceedings, I cannot do so to the destruction of life, or 
to any lasting inconvenience or injury. 
I can trace, then, the existence, superficial or deep, of a column 
reaching thus far down the spinal cord, and devoted to the 
functions of organic life. 
The External Respiratory Nerves . — I do not attribute any 
very great importance to the pectorales muscles, major and minor, 
or to the serratus magnus. The latter of these Sir Charles 
Bell calls the inferior external respiratory nerve of the thorax. 
It is given off from the fourth and fifth cervical nerves, and is 
distributed over the chest, on the serratus magnus anticus. This 
