124 PHYSIOLOGY AND HEALTH. 



tached to the back-bone and to the ribs, and fill all the 

 spaces between them. Some of these are attached by one 

 end to the spine, and, running obliquely forward and down- 

 ward, are attached by the other end to the ribs below. 

 Others are attached by one end to one rib above, and by 

 the other end to another rib below. When these muscles 

 contract, they lift the ribs. The posterior end of each rib 

 rolls in its socket in the spine; but the main portion of the 

 bone is raised and carried outward, and the whole cavity of 

 the chest is then expanded, in the same manner as the hoop 

 (Fig. XVI. a, c,) would allow the cavity of the cask to be ex- 

 panded, if the side c were lifted to the level of the side a, 

 which is supposed to be fixed. 



273. The ribs, spine, breast-bone, arid muscles (Fig. XV. 

 p. 121) bound the chest on all its sides. As this cavity is 

 conical, there is hardly any surface at the top. But there is 

 a broad and extensive surface at the bottom of the cone, 

 which is covered by a flat muscle, called the diaphragm. 

 (Fig. II. ?, p. 19.) This performs a part of the greatest 

 importance in the work of respiration. It is the flexible 

 partition that divides the chest from the abdomen, and sep- 

 arates the respiratory from the digestive organs. Its edges 

 are attached to the back-bone, and to the lower edge of the 

 lower ribs, to the breast-bone, and to all the lower part of 

 the chest. It forms an arch, upon the upper or convex 

 surface of which the lungs rest; and in the hollow below 

 some of the organs of the abdomen the liver, stomach, 

 &c. are placed. 



274. When the diaphragm is at rest, its arch points upward 

 into the chest, as the bottom of a common glass bottle is 

 turned into its cavity ; and its upper point reaches as high as 

 the fourth rib, and, consequently, must very materially lessen 

 the capacity of the chest, and press upon the lungs. But 

 when it is in action and contracted, the arch is drawn down, 

 and, leaving a space behind, enlarges the capacity of the 

 chest, and allows more room for the lungs to expand. The 

 diaphragm is the dividing-wall between the lungs and diges- 



