116 SKELETON. 



mensions of the thorax. The lungs, from being in a collapsed 

 and solid state, suddenly suffer an expansion of their cells by 

 the introduction of air into them, and increase twice or three 

 times in magnitude. This is accomplished by the elevation 

 of the ribs, and the consequent increase in the transverse dia- 

 meter of the thorax: it becomes a condition that for ever after- 

 wards remains, so that the lungs, even upon death, continue to 

 have their air cells distended, and do not return to a perfectly 

 collapsed state. The action of the diaphragm is but small in 

 the earliest periods of life, owing to the size and pressure of the 

 abdominal viscera against it; respiration is then principally car- 

 ried on by the elevation and depression of the ribs, and by their 

 being rolled outwards, a motion which the flexibility of their 

 cartilages and the looseness of their articulating surfaces favour 

 very much. 



At the age of puberty the thorax experiences a remarkable 

 augmentation. Its transverse diameter is sensibly increased, 

 and there is a general expansion of its volume, indicative of a 

 healthy and vigorous constitution. Should this not take place, 

 and the sternum be projected, it is supposed to mark a disposi- 

 tion to consumption. The enlargement of the thorax is un- 

 doubtedly also connected with a development of the organs of 

 generation at the same time. The exercise of the latter re- 

 quires greater vital powers than exist in early life, and the pro- 

 vision for it is manifested by the general increase of vigour and 

 firmness in the human frame; but it is not possible to point out 

 in what manner the sympathy exists, which, on the develop- 

 ment of the organs of generation, extends their influence to the 

 bony structure of the thorax. 



SECT. XI. OF THE MECHANISM OF THE THORAX. 



The thorax performs two very important offices in the ani- 

 mal machine: the first is to contain and protect the organs of 

 circulation and respiration, the second to assist in the function 

 of respiration, and perhaps that of circulation.* 



The mechanism of the thorax is such that the solidity of its 



* A very interesting paper, on this subject, lias lately been presented to the 

 French Institute by M . Barry. 



