172 



the qualities of the air must be suited to the condition of the vita! 

 power in the lungs, as those of the food, to the sensibility of the stomach. 



Being obliged, on this subject, to content myself with the ungracious 

 office of compiler, I hasten to bring this article to a close, and to refer the 

 reader for a fuller account of the air, considered in its physical and 

 chemical relations, to the works of M. M. Fourcroy, Hauy, Brisson, Sec. 

 to that of M. Guyton Morveau, on the method of purifying the air, when 

 from different combinations, it becomes unfit for respiration. 



LXXI. In man and in all warm-blooded animals, with a heart contain- 

 ing two auricles and two ventricles, the blood which has been conveyed 

 to all the organs by the arteries, and which has been brought back, by the 

 veins, to the heart, cannot return to it, without having previously passed 

 through the lungs, which are viscera destined to the transmission of air; 

 of a spongy texture, and through which the blood must, of necessity cir- 

 culate to get from the right to the left cavities of the heart. This course of 

 the blood constitutes the pulmonary or lesser circulation: it does not exist 

 in some cold-blooded animals. In reptiles, for instance, the heart has 

 but one auricle and one ventricle ; the pulmonary artery in them, arises 

 from the aorta, and conveys but a small proportion of the blood ; hence 

 the habitual temperature of these animals is much lower than that of 

 man. For the same reason too, there exist so small a difference, be- 

 tween their venous and arterial blood; the quantity of fluid vivified by 

 exposure to the air, in the pulmonary tissue, being too small to effect, by 

 its union with the general mass, a material change on its qualities. 



Mayow has given the most accurate notion of the respiratory organ, 

 by comparing it to a pair of bellows, containing an empty bladder, the 

 neck of which by being adapted to that of the bellows, should admit air 

 on drawing asunder its sides. The air, in fact, enters the lungs, only 

 when the chest dilates and enlarges, by the separation of its parietes, 

 agents of respiration are, therefore, the muscles which move the parietes 

 of the chest, these are formed of osseous and soft parts, in such a man- 

 ner, as to possess a solidity proportioned to the importance of the organs 

 which the chest contains, besides a capacity of motion required to carry 

 on the functions intrusted to them.* 



To carry on respiration which may be defined the alternate ingress of 

 air into the lungs, and its egress from those organs, it is necessary that 

 the dimensions of the chest should be enlarged (this active dilatation of 

 the cavity of the chest is called inspiration,) and that it should contract to 

 expel the air which it had received during the first process. This se- 

 cond action is called expiration, it is always of shorter duration than the 

 former, its agents are more mechanical, and the muscles have much less 

 influence upon it. 



The parietes of the chest are formed, at the back part, by the verte- 

 bral column, at the fore part by the sternum, and on the sides by the 

 ribs, which are osseo cartilaginous arches, situated oblique between 

 the vertebral column, which is fixed and becomes the point of support of 

 their motions, and the sternum which is somewhat moveable the spaces 

 between the ribs are filled by muscular planes of inconsiderable thick- 

 ness, the internal and external intercostal muscles, the fibres of which 



* See APPENDIX, Note U, for remarks on the mechanism of the respiratory 

 organs. 



