502 
MR. S1BSON ON THE MECHANISM OF RESPIRATION. 
Lateral muscles of locomotion (30). 
It may be affirmed that the numerous many-headed lateral muscles (30.30.30.) 
which pass from rib to rib, are devoted to locomotion. They, acting- on alternate 
sides, draw the ribs to which they are attached nearer to each other, curve the spine 
laterally, and spread the ribs of the opposite side farther apart. The other muscles 
probably serve both for locomotion and respiration. 
4. The ribs are simple. 
The ribs of the Snake are so simple that their action in breathing is very intelli- 
gible. If we distend the lungs to the full, and note how the position of the ribs is 
changed, what muscles are shortened and what lengthened, we shall ascertain what 
movements of the ribs will cause an inspiration, what an expiration, and we shall 
discover what muscles are inspiratory, what expiratory. The shortening of a 
muscle is conclusive evidence of its action ; if we find one set of muscles shortened 
when we distend the lungs, another set lengthened, we know that the shortened 
muscles are inspiratory, the lengthened expiratory. I have not ascertained expe- 
rimentally on the Snake that the same muscles act when the animal inspires that 
are shortened when we distend the lungs, but I have ascertained it in the Dog 
and the Ass. I observed these animals breathe when the muscles were exposed, 
noted the actions of the various muscles one by one, and after death inflated the 
lungs ; those muscles that acted during inspiration were invariably shortened when 
the lung was distended, and those that were passive were lengthened ; on the other 
hand, those muscles that were active during expiration, were shortened when the 
lungs were flaccid. This fact rendered it unnecessary to find out by further vivisec- 
tions the actions of the respiratory muscles, they could be ascertained leisurely on 
the dead body and represented by accurate drawings ; such drawings I have made 
by aid of the tracing-frame recommended to me by Dr. Hodgkin*. These drawings 
exhibit the changes effected in the form of the chest, the direction of the ribs, and 
the length of the various respiratory muscles in inspiration and expiration. 
In the Snake all the ribs have similar motions and similar muscles by which the 
motions are effected. 
5. On inspiration, or rather on distending the lungs, for I never noticed a snake 
inspire, the ribs are raised at their free extremity and throughout their whole course. 
The range of motion of the free end is greatest, that end being most remote from the 
centre of the ribs motion at the vertebrae. If we assume that the rib is a straight 
rod, we can examine the motions of the various ribs in relation to each other uncom- 
plicated by disturbing or diagonal motions. 
* This tracing-frame I have described in a paper on the Relative Situation of the Internal Organs, published 
in the volume for 1844 of the Transactions of the Provincial Medical and Surgical Association. It has been of 
essential value to me both in this inquiry and in that just referred to. 
