366 OF RESPIRATION. 



flat cells above mentioned ; indeed it would be impossible to conceive of a 

 method by which blood, whilst still retained within vessels, should be spread 

 over a larger surface for aeration. And if not restricted within vessels, 

 it could not be ceaselessly and rapidly driven on by the propulsive power 

 of the heart, which acts no less efficiently upon the pulmonary circulation 

 than upon the systemic, although the force exerted is much inferior, the 

 resisting power being far less, in consequence of the shortness of the circuit. 

 Estimating the surface of the capillaries at three-fourths of the whole sur- 

 face of the alveoli, a thin layer of blood of 150 square metres would be 

 exposed to the air, representing about two litres of blood ; and it has been 

 further calculated that it is renewed 10,000 times in the 24 hours (Kiiss). 1 

 Even making great allowance for error in these calculations, enough remains 

 to show how extensive is the surface for the interchange of gases between the 

 air and the blood. Two systems of vessels, the pulmonary and the bron- 

 chial, transmit blood, though in very different proportions, through the lungs. 

 In the former, which is by far the largest, the blood received from the sys- 

 tem, deficient in oxygen and charged with carbonic acid, is propelled by the 

 right ventricle through the pulmonary artery to the minute plexus of capil- 

 laries just described on the walls of the air-sacs and vesicles. From thence, 

 having undergone aeration, it is returned to the left auricle of the heart, 

 through the pulmonary veins, to be distributed by the left ventricle and 

 arterial system to the body at large. The smaller system, which has been 

 particularly examined by Prof. Turner, 2 consisting of the bronchial arteries 

 and veins, with numerous minute branches of the intercostal, internal mam- 

 mary, and other arteries; is distributed to the parts about the root of the 

 lung, the large bronchial tubes, and the pulmonic pleura ; some of its capil- 

 laries discharge themselves into the corresponding bronchial veins, whilst 

 others terminate in the pulmonary system, and, there is reason to believe, 

 play an important part in maintaining the circulation in cases where the 

 pulmonary artery has been obliterated by disease. The lymphatics of the 

 Lungs 3 arise either from branched cells in the alveolar septa, one of the 

 branches opening between the epithelial cells into the cavity of the alveoli 

 (pseudo-stomata of Klein) ; or, as Sikorski maintains, from the goblet-cells 

 of Schulze. In either case they form a plexus in the submucosa, and then 

 run with, and partially or wholly invaginate branches of the pulmonary 

 artery and vein. Cloetta found in the lung-tissue inosite, uric acid, taurin, 

 and leucin, besides the ordinary chemical constituents of the glandular tex- 

 tures, as albuminous compounds, collagen, chondrin, protagon, elastin, and 

 mucin. In the fetus, the tissue of the lungs contains a large number of cells 

 rich in glycogeu, which escape on pressure in the form of a milk-white fluid, 

 but which are not found except under pathological conditions in the adult. 



285. The contractility of the bronchial tubes (according to the experi- 

 ments of Dr. C. J. B. Williams)* may be excited by electrical, chemical, or 

 mechanical stimuli, applied to themselves. It is chiefly manifested in the 

 smaller bronchial tubes, those of less than a line in diameter having been 

 seen to contract gradually under the stimulus of galvanism, until their cavity 

 was nearly obliterated ; on the other hand, in the trachea and the larger 

 bronchi, the cartilaginous rings prevent any decided diminution in the calibre 

 of the tubes, and the muscular structure is much less distinct. It is doubt- 

 ful whether the contractility can be called into play by irritation of the 



1 Op. cit., 338. * Med.-Chir. Rev., 1865, p. 209. 



3 See Wywodzoff, VVien. Mod. Jnhrbucher, Bd. xi, p. 3; Sikorski, Ontralblutt, 

 f. d Med. Wiss., 1H70, p. 817; Klein, Proceed. Roy. Soc., Jan. 29th, 1874. 



4 Report of the Briti.-h Association for 1840, p. 411. 



