liyer and stomach against the under side of the diaphragm, 
and cause it to rise to its former height. 
332. u, The play of the ribs is rather more complicated. 
_ These bones, ¢ ¢, and ¢ ¢ (to the number of twelve on each 
_ side in Man), are attached at one end by a moveable joint to 
_ the spinal column, a; whilst at the other they are connected 
_ with the sternum (breast-bone) by an elastic cartilage. Now 
each rib, in the empty state of the chest, curves downwards 
ina considerable degree ; and it may be elevated by a set of 
- muscles, of which the highest, i, are attached to the vertebra 
of the neck and to the first rib, whilst others, ¢, ¢, e (termed 
intercostals), pass between the ribs. The cartilages also curve, 
downwards in the opposite direction, from their points of 
attachment to the sternum. When the breath is drawn-in, 
the first rib is raised by the contraction of the muscles, 7 ; and 
all the other ribs, which hang, as it were, from it, would of 
_ course be raised by this action to the same degree. But each 
of them is raised a little more than the one above it, by the 
contraction of its own intercostal muscle ; and thus the lower 
Tibs are raised very much more than the upper ones, Now 
_by the raising of the ribs, they are brought more nearly into 
a horizontal line, as are also their cartilages ; and since the 
pembaned length of the two is greater, the nearer they approach 
to a straight line, it follows that the raising of the ribs must 
throw them further out at the sides, and increase the pro- 
"jection of the sternum in front, thus considerably enlarging 
the capacity of the chest in these directions, When the 
movement of inspiration is finished, the ribs fall again, partly 
_ by their own weight, partly by the elasticity of their carti- 
| lages, and partly by the contraction of the abdominal muscles 
_which are attached to their lower border.—For the full under- 
standing of this description, it is desirable that the reader 
should examine the movements of his own or another person’s 
chest, by placing his fingers upon different points of the ribs, 
| and watching their changes of position during the drawing-in 
| and the expulsion of the breath. 
_ 333. Now the cavity of the thorax is itself perfectly 
slosed ; so that, if it were not for the expansion of the 
| tungs, a void or vacuum would be left when the diaphragm 
4s drawn down and the ribs elevated. The atmosphere 
around presses to fill that vacuum; but this it can only 
} _ RESPIRATORY MOVEMENTS OF MAMMALS. 283 
