407 
ON THE PHRENIC NERVE. 
It is stimulated by nervous influence derived from the phrenic 
nerve, and it contracts. In the act of contraction it is flattened, 
and the cavity of the thorax is enlarged, and the air rushes m 
through the nostrils and mouth, and the act of inspiration is 
performed. The nervous influence is then withdrawn, and tne 
muscle relaxes, and once more bellies into the thorax; and the 
cavity is diminished and the air is expelled, or, in other words, 
the act of expiration is performed. There are other auxiliaries 
of which I have spoken when treating of the function of respira¬ 
tion, but this is the main agent. The division of the phrenic 
nerve will not at once suspend respiration, for that will be carried 
on, painfully and laboriously, by the other muscles, which assist 
in raising and depressing the ribs; but, by degrees, they and the 
animal generally will be exhausted by their unaccustomed and 
violent action, and death will ensue. If the spinal chord is di¬ 
vided above the origin of the phrenic nerve, respiration imme¬ 
diately ceases, for all the nerves concerned in the dilatation and 
contraction of the chest are cut off. The cerebro-visceral may 
continue to influence the glottis and the lungs, but that will jc 
of little service when the parietes of the chest aie fixed. 
Whether a pure Organic Nerve. —It has been taken for granted 
by physiologists generally that the phrenic nerve is one of voluntary 
motion, and that the diaphragm is under the control of the will. 
In its usual action, the phrenic is a pure organic neive, and unin- 
fluenced by the will. The function of respiration,—the artenahza- 
tion of the blood,—is necessary to the animal from the moment of 
his birth; it is identified with the continuance of life ; and the con¬ 
tinued contraction and relaxation of the diaphragm, the main agent 
in the discharge of this function, is co-existent with the commence¬ 
ment of lif e, and ceases only when the vital principle is extinct. 
In its natural action, then, it is perfectly independent of the will. 
But is not the diaphragm occasionally a voluntary muscle ? Can 
we not increase or suspend its action when and how we please? 
Indirectly we can: we can call in other muscles to the aid of 
the diaphragm; we can give to them a certain velocity and 
power of action, with which the diaphragm from its relative 
situation must correspond. We can more fully and rapidly than 
usual expand the chest by the action of the intercostals, and also 
by that of the spinal accessory ; we can rapidly and violently 
contract it by the power of the abdominal muscles, or w>e can 
give fixidity to the chest; we can excite that kind of opposed or 
antagonist action between the muscles of inspiration and expna- 
tion, which the power of the diaphragm shall be insufficient to 
destroy, and between which it is in a manner paralyzed. tVe 
simplify the matter by considering the phrenic nerve as one of 
