240 PHYSIOLOGY. 



and the size of the cavity regulated ; 3, the respiratory organs 

 contained within the cavity) 



3. \The bones which enter into the composition of the 

 chest, are the sternum, or breast-bone ; twelve dorsal vertebrae, 

 and twenty four ribsf By examining the preceding plate, 

 Fig, 1., the sternum will be seen to terminate at its lower 

 part in a triangular piece of cartilage, which lies directly 

 over the stomach, and may be felt externally ;f the seven 

 uppermost, or true ribs, will be seen to extend the whole 

 distance from the spine to the sternum, strips of elastic car- 

 tilage being interposed between the breast-bone and their 

 anterior extremities, while the five lower, or false ribs, are 

 merely attached to each other by slips of cartilage ; the two 

 lower, indeed, float loosely, without any attachment to the 

 others, except by means of muscles?l 



4. I have already stated that the ribs are attached to the 

 spine at an acute angle, so that they cannot be moved out 

 of their ordinary position, without enlarging the dimensions 

 of the chest. The articulation of the ribs, both at the spine 

 and sternum, is effected by means of cartilage, thus allowing 

 sufficient degree of motion for the purposes of respiration in 

 a healthy state. The chest expands during inspiration for 

 the reception of air ; and during expiration, it contracts to 

 expel air, which is no longer useful. There are, therefore, 

 two motions required, an upward and an outward ; the first 

 increases the distance between the spine and sternum ; the 

 last, that between the ribs. Now these motions are both 

 effected by the ribs, and they are so articulated, that they 

 cannot perform the one without the other. By looking at 

 the cut, then, it will readily be seen how where the ribs rise, 

 the sternum will be pushed out, and of course the cavity of 

 the chest enlarged. 



5. The Muscles. All the muscles which are attached to 

 the bones just described, aid more or less in respiration, 



Ithough the intercostal muscles are those chiefly concerned^ 

 These run from one rib to another, filling up the spaces be- 



