2<}o Mr. Bell on the nerves which associate the muscles 
ribs ; but it cannot exert this power, independently of the 
trapezius, since, without this combination, its force would be 
exerted in moving the scapula, and not the ribs ; unless the 
scapula be fixed, or pulled back by the trapezius , the serratus 
is not a muscle of respiration. 
In this manner do these three powerful muscles hang to- 
gether in their action, combining with the diaphragm to en- 
large the cavity of the chest in all its diameters. 
The course of our enquiry leads us to ask, are these muscles 
privileged above others by any peculiarity of nerves ? And 
the answer is plain : to these muscles alone, are the nerves, 
which I am about to call respiratory nerves of the chest, dis- 
tributed. 
Anatomy of the respiratory nerves of the trunk * 
The nerves which give rise to the extraordinary intricacy 
of this system on the side of the neck, are the spinal accessory 
nerve, the phrenic nerve, and the external thoracic nerve. 
By reference to any common book of anatomy, the phrenic 
nerve (4, Fig. 2.) will be found to have its great root or origin 
from the fourth cervical nerve ; and there joins this, a more 
slender branch from the third cervical nerve. But, besides 
these roots, it has connections, which of themselves would 
mark the relations of the nerve ; high in the neck, it is con- 
nected with the nervus vagus and with the lingualis medius, while, 
at the same time, a branch is given off to the muscles of the 
larynx. The trunk of the nerve descends into the cavity of the 
thorax, and gives no branches until arriving at the diaphragm 
(Fig. 2.G), it sends out numerous diverging branches, which 
are lost in the substance of that muscle. 
* See Plate XXXI. Fig. I and 2. 
