, THE CRANIUM. 121 



the whole periphery of the thorax: and its cavity being enlarged 

 by the contraction of the several muscles appropriated to the 

 elevation of the ribs; the moment this contraction ceases, the 

 latter are impelled downwards. From all this it will be under- 

 stood that the muscles, by creating a vacuum in the lungs, cause 

 the vacuum to be filled by the introduction of air through the 

 trachea; and upon their ceasing to contract, the several agents 

 mentioned cause the expulsion of the same air. It is generally 

 believed, that the surface of the lung is every where in contact 

 with the thorax; it appears, however, doubtful, whether there 

 is not a space between the pleura pulmonalis and diaphragmalis, 

 particularly at the most posterior and inferior part of the dia- 

 phragm. Certain it is, that adhesions there, are much less com- 

 mon than in other parts of the thorax. 



The ligaments at the spinal extremities of the ribs, by being 

 put on the stretch in inspiration, have also some tendency to 

 throw down the ribs in expiration. In short, the contraction 

 of the thorax may be set down as the result of the joint action 

 of the atmosphere, the cartilages of the ribs, the ligaments, the 

 contraction of the lungs, and the muscles. When the structure 

 of the lung is so altered that its elasticity is impaired or de- 

 stroyed, expiration becomes then much more difficult. 



CHAPTER II. 



OF THE HEAD. 



THE head is placed upon the upper extremity of the verte- 

 bral column, and consists in a considerable number of bones, 

 which are either in pairs, or, if single, have the two sides sym- 

 metrical. Some of these bones form a large cavity, the cranium, 

 for containing the brain; the others are employed in the forma- 

 tion of the nose; of the orbit for the eye-ball; and of the mouth. 

 The head, for the most part ovoidal, presents very striking va- 

 rieties of form between different individuals and different na- 

 tions. It is thought by physiologists, that the moral or intel- 

 lectual condition of a people, their habits, climate, and food, 



VOL. I. 11 



