118 SKELETON. 



are inflicted directly on it. Each rib represents an arch, the 

 summit of which is its centre, and the base its two extremities. 

 The abutments of the base are, the sternum at one end and the 

 spine at the other : a displacement from them is completely pre- 

 vented by the strength of the ligamentous attachments, as well 

 as by the form of the surfaces. Under these circumstances, as 

 fracture occurs preferably to dislocation, it is generally at the 

 point stricken. 



The abdominal or false ribs, from their want of attachment 

 to the sternum, present a very different condition. Their an- 

 terior extremities, therefore, yield readily, and are driven in- 

 wards towards the abdomen. 



The second function of the thorax, relating to its influence on 

 respiration, is executed by its dilating and contracting, whereby 

 the air is received into, and expelled from it. The spine is the 

 fixed point for the motions of the ribs in respiration. In the act 

 of dilatation, the capacity of the thorax is augmented in three 

 directions, vertically, transversely, and antero-posteriorly, or 

 from the sternum to the spine. The vertical augmentation is ac- 

 complished by the diaphragm ; and, as mentioned, is much great- 

 er proportionally in the adult than in the infant, from the greater 

 comparative size of the abdominal viscera in the latter. The 

 transverse augmentation is produced by the successive contrac- 

 tion of the intercostal muscles, which raise the ribs upwards. 

 The first rib is moved inconsiderably, in consequence of its short- 

 ness and of its continuity with the sternum. The attachment of 

 the scaleni muscles to its upper surface, serves rather to give a 

 fixation to it, and to prevent it from being drawn down by the 

 other ribs, than to produce by their contraction an elevation of 

 it. The first rib may, therefore, be considered as a fixed point. 

 The first intercostal muscles contracting from it, draw up the se- 

 cond rib, which, in its turn, becoming a fixed point for the second 

 intercostal muscles, they contract and draw up the third rib, and 

 so on successively to the last. It is the obliquity of the ribs from 

 behind, downwards and forwards, which enables this elevation of 

 them to produce an increase in the lateral diameter of the thorax: 

 without such obliquity, their elevation would not have the ef- 

 fect. But the obliquity alone could be of but little service, if 



