122 PHYSIOLOGY AND HEALTH. 



266. The chest (Fig. XV.) extends from the neck to the ab- 

 domen. It is conical in shape, being small at its upper end, 

 and larger at the lower part. Jt is enclosed by bone at the top 

 and on its sides, and by muscle at the bottom. The breast- 

 bone,' .a, (Fig. XV. p. 121,) is in front; the spine, b. b, or 

 back-bone, is behind ; and the ribs, c, c, c, cover the sides of 

 the chest. 



267. The spine, or back-bone, is composed of twenty-four 

 bones, called vertebra, which are connected by thick layers 

 of very strong and elastic cartilage, or gristle, between them. 

 These give to the column great flexibility and freedom of 

 motion, and such strength that, through all the chances of 

 accidents and violence, these bones are very rarely broken or 

 displaced. Yet it may be bent in any direction, and is capable 

 of sustaining great burdens that may be placed upon the head. 

 Twelve of these bones form part of the chest, and to these 

 twelve vertebrae or bones of the back are attached twenty-four 

 ribs, twelve on each side. 



268. The breast-bone, a, is thin and flat, reaching from the 

 neck to the region of the stomach. It is covered with so little 

 flesh as to be perceptible to the touch. At the lower end is 

 attached a cartilage or gristly substance, that extends about 

 two or two and a half inches downward, and ends in a point 

 at the pit of the stomach. This breast-bone forms the front 

 pillar of the chest, though by no means an immovable one, for 

 it rises and falls with all the motions of the ribs. 



269. The ribs, c, c, c, compose the principal part of the 

 framework of the chest. They surround the cavity from the 

 back to the breast-bone, covering all the sides and most of 

 the anterior and posterior portions of the cavity. All are 

 joined to the back-bone by their posterior end, and ten of 

 them are connected with the breastrbone by their anterior 

 nds. Some of these are fixed directly to the breast-bone; 

 others terminate in cartilages of an inch or more in length, 

 that extend to the breast-bone ; by which arrangement these 

 ribs have a great freedom of motion. 



270. The ribs nearly surround the cjiest, somewhat as 



