and functions of the nerves. 405 
If the nerves be exposed in a living animal, those of this 
class exhibit the highest degree of sensibility ; while, on the 
contrary, nerves not of this original class or system, are 
comparatively so little sensible, as to be immediately distin- 
guished ; in so much that the quiescence of the animal sug- 
gests a doubt whether they be sensible in any degree what- 
ever. If the fifth nerve , and the portio dura of the seventh , be 
both exposed on the face of a living animal, there will not 
remain the slightest doubt in the mind of the experimenter 
which of these nerves bestows sensibility. If the nerve of 
this original class be divided, the skin and common substance 
is deprived of sensibility ; but if a nerve not of this class be 
divided, it in no measure deprives the parts of their sensi- 
bility to external impression. 
More particularly of the respiratory nerves. 
The nerves which connect the internal organs of respira- 
tion with the sensibilities of remote parts, and with the 
respiratory muscles, are distinguished from those of which 
we have been speaking by many circumstances. They do 
not arise by double roots; they have no ganglia on their ori- 
gins ; they come off from the medulla oblongata and the upper 
part of the spinal marrow; and from this origin, they diverge 
to those several remote parts of the frame which are com- 
bined in the motion of respiration. These are the nerves 
which give the appearance of confusion to the dissection, 
because they cross the others, and go to parts already plenti- 
fully supplied from the other system. 
The following are the nerves to be enumerated as respi- 
ratory nerves, according to their functions. 
