618 MR. youatt’s veterinary lectures. 
and trapezius muscles on that side ceased to act, yet respiration 
was violently although imperfectly performed by the intercostals 
and the other respiratory muscles, the nerves of which had not 
been divided. 
Again, when the spinal marrow at the lower part of the neck, 
and beneath the perfect formation of the phrenic nerve, was 
divided, respiration was continued by means of the diaphragm. 
In another case the phrenic nerves were first divided, and then 
the spinal marrow at the bottom of the neck. Respiration was 
stopped in the chest; but there was a catching and strong action 
at intervals of the associate respiratory muscles of the nostrils, 
and the side of the chest. When this animal was apparently 
dead, he was reanimated by artificial breathing, and these 
muscles again contracted, but the chest remained at rest. 
The Diaphragm the principal Agent in Respiration. —These 
experiments are decisive, and mark the diaphragm as the main 
agent in the work of respiration. The others are mere auxiliaries, 
little needed in ordinary respiration, but affording the most im¬ 
portant assistance, when the breathing is more than usually 
hurried. 
The Mechanism of Respiration . —Then the mechanism of 
respiration may perhaps be thus explained. I will suppose the 
lungs to be in a quiescent state. The act of expiration has been 
performed, and all is still and quiet. From some cause enveloped 
in mystery—connected with the will, but independent of it—some 
stimulus of an unexplained and unknown kind—the phrenic 
nerve acts on the diaphragm, and that muscle contracts; and, by 
contracting, its convexity into the thorax is diminished, and the 
cavity of the thorax is enlarged. At the same time, and by 
some consentaneous influence, the intercostal muscles act, with 
no great force indeed in undisturbed breathing, but in proportion 
as they do act the ribs rotate on their axes, their edges are 
thrown outward, and thus a twofold effect ensues ;—the posterior 
margin of the thorax is expanded, and the cavity is plainly en¬ 
larged $ and also by the partial rotation of every rib the cavity is 
still more increased ;—at the same time, this bellying out of the 
parietes of the chest is an antagonist to, and limits the too 
powerful contraction of, the diaphragm. 
Auxiliary Muscles. —By some consentaneous influence, the 
spinal accessory likewise exerts its power, and the sterno-maxil- 
laris is stimulated by the anterior division, and the motion of the 
head and neck corresponds with and assists that of the chest ; 
while the posterior, by its anastamoses with the motor nerves of 
the levator humeri and the splenius, and many other of the 
muscles of the neck and the shoulder, and by its direct influence 
