394 



RESPIRATION. 



[CHAP. xxix. 



meshes of the net work are not much wider than the vessels them- 



seves. 



Mr. Rainey says that on the surface of the lung, the capillary 



Fig. 207. 



View of a thin section of the lung of a cat, which had been injected by the pulmonary 

 artery with gelatine, so as to fill blood-vessels and air-cells, and had been sliced when cold. 

 a a a. Air-cells and lobular passage in section. 6 b. Their wall in section, c. Their wall in 

 face. d. Extremely faint nucleus in the same, e e. Capillaries, h. Nucleus in wall of capil- 

 lary, n. Small pulmonary artery or vein with simple wall. Magnified 250 diameters. 



network extends over a space corresponding to not more than ten 

 or twelve cells, i.e. over from one-thirtieth to one-twentieth of an 

 inch, and if the rate of the blood be taken at an inch and three 

 quarters per minute, according to the estimate of Valentin, drawn 

 from observation of the frog's foot, the blood would be somewhere 

 about one second and a half in contact with the air at each circuit. 

 But probably in the higher animals its transit is more rapid. 



We may here refer to some very interesting observations on the 

 arrangement of the capillaries and terminal air passages, lately 

 made by Mr. Rainey. This careful and accurate anatomist was 

 the first to insist on the fact that, in mammalia and birds at least, 

 the capillaries are, as we have above noticed, bare ; exposed on 

 their exterior to the air, not covered by any membrane, either 

 mucous or serous, or by any epithelium ; and that by this arrange- 

 ment the most perfect aeration of the blood which traverses them 

 is provided for. The air and the blood may be said to be in 

 contact through the delicate capillary wall, a film less than 



