617 
THE DIAPHRAGM, &C. 
The Nerves. —The functional nerve of the diaphragm, or that 
from which it derives its principal action, and which consti¬ 
tutes it a muscle of respiration, is the phrenic or diaphragmatic. 
Although it does not proceed from that portion of the medulla 
oblongata which gives rise to the glosso-pharyngeus and the 
par vagum, yet there is sufficient to induce us to suspect that it 
arises from and should be referred to the lateral column between 
the superior and inferior, the sensitive and motor nerves, and 
which may be evidently traced from the pons varolii to the very 
termination of the spinal chord. This specimen illustrates their 
lateral origin. They do not spring from the lateral column so 
decidedly as the spinal accessory, and in one continued line, but, 
as I shall more particularly shew you when treating of the sen¬ 
sorial system, there is a peculiarity in their origin, and espe¬ 
cially from the inferior surface of the chord, which would almost 
justify us in the conclusion, that, connected with the respira¬ 
tory nerves in function, they are derived from the same 
common source. 
The phrenic nerve first appears considerably lower in the neck 
in all our patients than in the human being. The precise situa¬ 
tion is uncertain. I have sometimes traced a filament from the 
fourth cervical nerve, which proceeds downward, and unites with 
branches from the fifth and sixth, to constitute a perfect 
nerve; but in other cases there has been a filament from the fifth, 
and the nerve has been principally composed of branches from 
the sixth and seventh cervicals. The more extensive motion in 
the neck of the quadruped, or the exposure of the superior part 
of it to injury, might cause a higher origin of the phrenic nerve 
to be inconvenient or unsafe; but, however, being formed, it 
travels down the neck without any anastomosis ; it enters the 
chest between the two laminae of the mediastinum ; it passes 
over the pericardium, where it is a beautiful object in a dissec¬ 
tion of the chest; and, still pursuing its lonely course, arrives at 
the centre of the diaphragm, and ramifies over the whole sub¬ 
stance of that muscle. 
Experiments on the Phrenic Nerve .—Sir Charles Bell has in¬ 
stituted some interesting and satisfactory experiments on the 
function of this diaphragmatic nerve ; and he has collected some 
others, at which I will briefly glance. When the phrenic nerves 
were divided in an ass, a remarkable heaving of the chest took 
place. The other auxiliary respiratory muscles, and particularly 
the intercostals, were labouring hard to supply the deficiency of 
the diaphragm ; and the chest rose higher than usual, and its 
margins particularly were more expanded at each inspiration. 
The spinal accessory was then divided on one side; the mastoid 
VOL. V. 4 o 
