A PAIR OF LIVING BELLOWS 125 



When the left arm and shoulder are cleared away the 

 side wall of the thorax is laid bare from the neck above 

 to the loins below. We note that from top to bottom 

 the wall is strengthened by ribs — a series of bent bony 

 levers of a remarkable kind. These levers are arranged 

 in a definite order ; they become longer as one passes 

 from the 1st to the 8th, and then shorter until the 12th 

 is reached, which may be no longer than a little finger. 

 When the body is in the upright posture not even the 

 first rib is horizontal ; all of them, as they pass towards 

 the front of the body and the breast-bone, slope down- 

 wards to a greater or less degree, the downward slope 

 increasing as we descend the series. We may dismiss the 

 two last ribs of the series — the 1 ith and 12th — from our 

 consideration now, for they take only an indirect part in 

 expanding the thorax. They end abruptly, after passing 

 forwards for some distance, in the muscles of the body- 

 wall. When a breath is taken these two ribs remain 

 stationary or may actually move downwards, whereas the 

 other ribs swing upwards. All the ribs, except the two 

 last, when they reach the front wall of the thorax undergo 

 a sudden change in substance ; bone is replaced by gristle 

 or cartilage — a material which has the springiness and elas- 

 ticity of cane and gives a resiliency to the thoracic bellows. 

 By means of these cartilaginous continuations the upper 

 seven ribs are attached directly to the breast-bone, thus 

 forming half-hoops ; we may regard the breast-bone as 

 forming a small part of the load which the rib-levers have 

 to lift when they swing upwards in the act of inspiration. 

 The 8th, 9th, and 10th ribs also help in this act, for 

 although their cartilages do not directly reach the breast- 

 bone yet they do so indirectly, for the cartilage of each 

 turns up and joins the one directly above it, and thus all 

 help to support and lift the breast-bone in front. 



In this way the lateral wall of the thoracic bellows is 

 strengthened and made movable by the insertion of a 

 series of bent levers. There is no need to study the 

 whole series of ribs ; if we know about the mechanism of 

 one of them, we understand that of all. The 7th rib will 



