RESPIRATORY MOVEMENTS OF MAMMALS. 283 



liver and stomach against the nnder side of the diaphragm, 

 and cause it to rise to its former height. 



332. ii. The play of the ribs is rather more complicated. 

 These bones, c c, and c c' (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 

 in a considerable degree ; and it may be elevated by a set of 

 muscles, of which the highest, i, are attached to the vertebrae 

 of the neck and to the first rib, whilst others, e, e, 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, i ; 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 

 ribs 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 

 combined 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 

 closed; so that, if it were not for the expansion of the 

 lungs, a void or vacuum would be left when the diaphragm 

 is drawn down and the ribs elevated. The atmosphere 

 around presses to fill that vacuum; but this it can only 



