62 PHYSIOLOGY. 



surrounded, as in the animal which has respired, by plexuses of 

 blood-vessels. 



The chemical process of respiration is not essentially dependent 

 on the respiratory movements. They merely serve to expel the air, 

 (or water) which has undergone the change induced by the chemical 

 process constantly carried on between it and the blood, and to renew 

 the supply of fresh air or water. 



The lungs, by their internal surface, offer an immense expansion 

 for the action of the blood and air on each other; and, as they are 

 never completely emptied by the act of expiration, this action is con- 

 stant. By the contraction and dilatation of the chest, the motions of 

 which the lungs follow, a portion of the altered contents of the pul- 

 monary reservoir is first expelled, and then a new supply introduced, 

 to undergo change in its turn. 



According to the inquiries of M. Bourgery the development of the 

 air-cells continues up to the age of thirty, at which time the capacity 

 of respiration is greatest ; it subsequently decreases, especially in 

 persons who suffer from cough. The violence of such expiratory 

 efforts frequently causing rupture of the air-cells, thus gradually 

 producing that emphysematous condition of the lungs so common in 

 elderly people. The power of increasing the volume of air by a 

 forced inspiration is much greater in young than in old persons, and ^ 

 is twice as great in males as in females of the same age, a circum- 

 stance which is evidently connected with the extent to which mus- 

 cular efforts can be carried in these classes respectively. 



Movements of respiration. — These may be divided into two 

 classes, those of i^ispiration and those of expiration. By the first 

 is meant the action by which air enters the lungs ; by the latter, the 

 act of expelling from the lungs the air received in inspiration. 



In mammalia generally, as well as in man, inspiration and expi- 

 ration are performed by the dilatation and contraction of the cavity 

 of the thorax. As soon as the walls of the chest are drawn wide 

 asunder, and the thorax dilated, the external air rushes through the 

 trachea and its branches into the air-cells, distending them in pro- 

 portion to the dilatation of the thorax, thus keeping the surface of 

 the lung accurately in contact with the thoracic walls in all their 

 movements. This contact, however, can only take place while the 

 thoracic cavity is closed on all sides, so that the air cannot exert any 

 pressure on the outer surface of the lung, by which that upon the 

 inner would be balanced. Hence it is, that in penetrating wounds 

 of the chest, the lungs cannot be fully distended by inspiration, be- 

 cause the air entering through the wound into the cavity of the 

 pleura balances the pressure of the air on their inner surface ; the 

 lungs, in such a case, remain collapsed, although the thoracic 

 parietes dilate. 



Inspiration. — The diaphragm contributes the principal share to 



