MECHANISM OF THE THORAX. 117 



materials, and its rounded shape, present a very efficacious de- 

 fence to its viscera from the influence of blows on its outside. 

 The effects of the latter are also materially diminished by the 

 thickness and contraction of the several large muscles which are 

 placed on its surface. On its back part the thick longitudinal 

 muscles of the spine, as well as those running to the superior 

 extremities, fill up the gutters on each side of the spinous pro- 

 cesses, and make a fleshy protuberance, divided into two by the 

 raphe which extends the length of the back over the spinous 

 processes. In front it is less protected, owing to the sternum 

 being immediately under the skin. Nevertheless, when blows 

 are inflicted on this part, their effects are much diminished by 

 the elasticity of the cartilages of the ribs, and by the direction, 

 obliquely downwards, of the ribs themselves; both of which 

 dispose the sternum to retreat backwards, and to yield to the 

 impelling force. The recession will take place more readily at 

 the moment of expiration; and when the muscles which elevate 

 the ribs are not on their guard. In those deliberate exertions 

 of the strength of the thorax, exhibited by individuals lying 

 down on their backs, and sustaining a heavy weight on the ster- 

 num, the ribs are saved from injury by different means. The 

 arched form, itself, of the front of the thorax, is of considerable 

 service in the resistance under such circumstances ; this, how- 

 ever, would be easily overcome, and the ribs would break, if 

 the arch were not sustained in its elevation by the contraction 

 of the large muscles on its sides ; as the serratus major, the pec- 

 toralis major and minor, each of which, by acting on the de- 

 pressed anterior extremities of the ribs and their cartilages, has 

 a tendency to keep them elevated. Fractures of the ribs, from 

 blows or force applied in front, are not so liable to occur in the 

 part stricken as in the point feeling the greatest momentum, 

 which from the semicircular form of the ribs is in or near their 

 middle : this exhibits a true example of what the French writers 

 call the contre-coup. Bichat says, that the fracture by contre- 

 coup is much more common when the individual being struck 

 unexpectedly, has not had time to throw his muscles into a state 

 of contraction, for the protection of the ribs. 



The lateral convexity of the thorax being greater than that 

 in front or behind, and having the same assistance from the 

 muscles mentioned, presents a stronger resistance when blows 



