516 MR. youatt’s veterinary lectures. 
depression or channel, in which run both bloodvessels and nerves. 
The space between them is filled up by muscular substance firmly 
attached to the borders of the ribs. There are said to be two 
layers of muscles, the fibres of which cross each other in the di¬ 
rection of an X, the direction of those of the outer layer being 
oblique anteriorly-posteriorly, and of the inner layer also oblique 
but posteriorly-anteriorly. 
lieason of their Oblique Direction .—There is a manifest reason 
for this: if the fibres ran straight across from rib to rib, they 
might act powerfully, but their action would be exceedingly 
limited. A short muscle can contract but a very little way, and 
only a slight change of form or dimension can be produced. 
By running diagonally from rib to rib these muscles are double 
the length that they could otherwise have been; and the extent 
of the contraction is doubled, and thus the ribs are brought 
closer to each other than they could otherwise have been. It is 
a general rule with regard to muscular action, that the power of 
the muscle depends on its bulk, and the extent of its action on 
length. 
The Rotatory Motion of the Ribs enlarging the Thorax. —Well 
then, gentlemen, I will suppose that from the influence of the 
intercostal nerves, these muscles contract, and the ribs are brought 
closer to each other; and as the two first ribs are, in a manner, 
the fixed point, the bony walls of the chest are brought forward 
while they are shortened. The effect of this, you would say, would 
be to contract the cavity of the chest. Not so. The motion of 
the rib, as depending on the two joints by which it is connected 
with the spine, is purely a rotatory one. The muscles are in¬ 
serted, not only into the edges of the ribs, but to a little distance 
from the edges; and the effect of the action of the muscles is to 
rotate the ribs more than to approximate them, and in thus ro¬ 
tating them, to throw them outward, and thus, in fact, to expand 
and not to contract the cavity of the chest. In the action of the 
intercostals, we are probably not to consider it a simple contrac¬ 
tion of the centre of muscle, equally affecting the rib before and 
the rib behind, but we are to regard the posterior edge of the rib 
as the fixed point, the anterior of the next rib as the moveable 
one, and thus, while they are approximated to each other by the 
contraction of the muscle, they are more rotated and drawn out¬ 
ward, and the cavity of the chest enlarged. This is particu¬ 
larly the case with the posterior ribs, which are not tied down to 
the sternum, and are much more loosely attached to each 
other, and therefore have greater freedom of motion, and are ro¬ 
tated to a greater extent, and thrown more outward. More espe¬ 
cially is this the case with the two last ribs, which have only one 
