348 Dr. Russell and Mr. Home on the Expansion 
of bending equally with the other ribs towards the belly, they 
go out in a lateral direction, having only a slight curvature, and, 
when depressed, lie upon the side of the spine, on one another. 
The first rib is shorter than the rest; and they become gra- 
dually longer to the tenth and eleventh, which are the longest ; 
they afterwards become gradually shorter to the twentieth, 
which is nearly of the same length as the first; so that the ribs 
on each side, when extended, form ail oval figure, of which the 
spine is the middle line or long axis. 
In the extended state of the ribs, the skin of the back is 
brought over them, forming the hood ; and, in their depressed 
state, the hood disappears. 
The ribs are raised by four sets of muscles : one set, from the 
spine to the upper edge of each rib ; a second set, from the ribs 
above, passing over two ribs to the third rib below ; another set 
have their origin from the rib above, pass over one rib, and are 
inserted into the second below ; and a fourth set pass from rib 
to rib. 
The combined effect of these four sets of muscles, raises and 
extends the ribs : their direction and appearance is so distinctly 
seen*in the annexed Figures, as to make a more particular de- 
scription, in a Paper of this kind, unnecessary. 
The skin of the back is brought forwards on the neck, by a 
large set of very long muscles, going off from each of the first 
twenty ribs on each side, a quarter of an inch from their head, 
by a tendinous origin, which soon becomes fleshy ; the longest 
of these muscles is two inches long ; they are inserted into the 
skin, and, when the ribs have been first extended, have the power 
of bringing the skin forwards to a great extent. 
By these means, the hood is formed. 
