of the chest in breathing, speaking, and expression. 295 
external thoracic nerves in fishes, the order of their muscular 
system not requiring them. In birds, the structure of the 
wing, and the absence of the mastoid muscle, render the 
spinal accessory nerve unnecessary ; it is wanting for the 
reason that in the absence of the diaphragm there is no 
phrenic nerve. Quadrupeds have the three respiratory nerves 
of the trunk ; but even in them there are variations in the 
muscular frame, which illustrate the appropriation of the 
nerves. The construction of the neck of the camel is like that 
of birds ; there is a succession of short muscles along the side 
of the neck, and attached to the vertebras ; but there is no 
long muscle, like the sterno-cleido-mastoideus , contributing to 
the motion of respiration. There is, accordingly, no spinal 
accessory nerve in the neck of this animal. 
We have a remarkable example of the manner in which 
these nerves vary in their course of distribution, and yet re- 
tain their appropriate functions, in the nerves of the neck of 
birds. In them, the bill precludes the necessity of the portio 
dura going forward to the nostrils and lips ; the nerve turns 
backwards, and is given to the neck and throat ; and it is 
particularly worthy of remark, that the action of raising the 
feathers of the neck, as when the game cock is facing his 
opponent, is taken away by the division of this nerve. 
The functions of these nerves farther illustrated. 
Before having recourse to experiments on brutes, we may 
observe what takes place in our own bodies By placing the 
hand upon the neck, we may be sensible that the mastoid 
muscle has two motions. The lower extremity of the muscle 
is fixed when we move the head ; but when we use the muscle 
