DEVELOPMENT OF THE THORAX. 115 



SECT. X. OF THE DEVELOPMENT OP THE THORAX. 



In the foetus the shape of the thorax differs much from that 

 of the adult, in the greater comparative extent of its antero-pos- 

 terior diameter, and in the projection of the sternum. The state 

 of the thoracic viscera, at this period, calls for such an arrange- 

 ment; as the heart and thymus gland, which are in the middle, 

 have a considerable extent, whereas, the lungs are still collapsed 

 from the emptiness of their air cells. The ribs are but little 

 curved at their posterior parts, the angle being by no means 

 well formed, in consequence of which, the fossa on each side of 

 the bodies of the vertebrae, in the thorax, is not so deep; neither 

 is the fossa behind, on each side of the spinous processes, so 

 fully marked. The superior opening of the thorax is more 

 round from the increase of the antero-posterior diameter. The 

 inferior opening is extremely large, both from the elevation of 

 the sternum, and from the pressure of the abdominal viscera, of 

 which the liver, from its great extent, is a principal agent. The 

 vertical diameter of the thorax is % small, from the ribs, particu- 

 larly the lower ones, being pressed up one against the other, by 

 the diaphragm, acted on by the abdominal viscera. 



The bones individually are in the following state at birth. 

 The ribs are almost completed, the heads, where they join the 

 spine, being in a state nearly as perfect as at any subsequent pe- 

 riod of life, and not by any means in the condition of a cartila- 

 ginous epiphysis, as is presented in the extremities of the cy- 

 lindrical bones generally. These bones, as Bichat very justly 

 observes, are destined to a function which commences imme- 

 diately upon birth, and which requires in them as much perfec- 

 tion then, as they have in the adult. For respiration is different 

 from locomotion, the latter requires a species of education, which 

 may be given gradually, whereas one respires from the begin- 

 ning as he will respire always. The sternum, which is less im- 

 mediately connected with breathing, and only contributes to 

 the general solidity of the thorax by completing its circumfe- 

 rence, is in a state almost cartilaginous, and presents only nuclei 

 of ossification in its several pieces. 



At the instant of birth, a great change is produced in the di- 



