EXHALATION AND ABSORPTION BY THE LUNGS. 411 



blood in the capillaries of the lungs is not due (as some have supposed) to a 

 mechanical impediment: but the pressure is immediately increased by the 

 admission of atmospheric air, which occasions the renewal of the pulmonary 

 circulation, and the consequent increase in the supply of aerated blood to the 

 systemic arteries. It has been recently shown by Mr. Wharton Jones,* that 

 the capillary circulation in a frog's foot is retarded or even checked by the 

 direction of a stream of carbonic acid gas against the membrane ; and he attri- 

 butes this stagnation to the disposition thus produced in the red corpuscles, to 

 aggregate together and to adhere to the walls of the vessel so as to choke up 

 its calibre. 



IV. Exhalation and Absorption by the Lungs. 



549. The alteration in the proportions of its usual gaseous ingredients, is 

 by no means the only change which the blood undergoes in the Lungs. It 

 parts, also, with a considerable amount of water, in the form of vapour; this 

 usually contains a certain proportion of animal matter; and it is sometimes 

 charged with volatile substances, which have been elsewhere introduced into 

 the blood, or which have been formed during its assimilation. It may also 

 absorb from the atmosphere volatile matter diffused through it. Both these 

 changes are probably to be explained upon simple physical principles ; bein^ 

 dependent on the exposure of the blood to the atmosphere, with a very exten- 

 sive surface, and through a membrane of great permeability. Of the fluid 

 ordinarily exhaled with the breath, a part doubtless proceeds from the moist 

 'lining of the nostrils, fauces, &c. ; but it is indisputable that the greater pro- 

 portion of it comes from the lungs ; since, when the respiration is entirely 

 performed through a canula introduced into the trachea, the amount of watery 

 vapour which the breath contains is still very considerable. The quantity 

 which thus passes off is by no means trifling ; probably between 16 and 20 

 ounces in the twenty-four hours. It is not so liable to variation under the 

 influence of temperature, the movement of the surrounding air, and other 

 similar causes, as is the cutaneous transpiration ; for air, which has found its 

 way into the air-cells of the lungs, will, under almost all circumstances, be 

 nearly the same in regard to such conditions, and will, therefore, dissolve an 

 equal amount of watery vapour. It is considered by Dr. Prout, that the prin- 

 cipal source of this vapour is not the blood properly so called, but the chyle 

 and lymph which have just been introduced into it from the thoracic duct ; a 

 loss of a portion of their fluid being required to give them sufficient concentra- 

 tion. A process very analogous takes place in Plants ; for a very large pro- 

 portion of the water taken up in the crude sap, is parted with in the leaves. 

 But it is probable that a part, at least, of the water thrown off by the lungs is 

 generated by the union of Oxygen and Hydrogen, during the course of the 

 Circulation ( 533). The fluid thrown off from the Lungs is not pure water. It 

 holds in solution, as might have been expected, a considerable amount of car- 

 bonic acid, and also some animal matter; the exact nature of the latter, which, 

 according to Collard de Martigny, constitutes about 3 parts in 1000, has not 

 been ascertained. If the fluid be kept in a closed. vessel, and be exposed to 

 an elevated temperature, a very evident putrid odour is exhaled by it. Every 

 one knows that the breath itself has, occasionally in some persons, and con- 

 stantly in others, a fetid taint ; when this does not proceed from carious teeth, 

 ulcerations in the air-passages, disease in the lungs, or other similar causes, it 

 must result jfzm the excretion of the odorous matter, in combination with 

 watery vapour, from the pulmonary surface. That this is the true account of 



* Brit, and For. Med. Rev., vol. xiv. p. 600. 



