230 



RESPIRATION. 



Fig. 244. a, Rudiment of the 

 lung in the embryo of the fowl 

 of the fourth day ; 6, the lung 

 in the embryo of the sixth day. 

 Both figures twice the size of 

 nature. 



bits a peculiar structure, which may be studied very readily in 

 the lungs of the live newt (fig. 

 230), or in preparations of the 

 same part that have been finely 

 injected. From the whole extent 

 of the pulmonary artery a vast 

 number of very small arteries arise, 

 the orifices of which give the inner 

 surface of its principal branches 

 the appearance of a regularly per- 

 forated sieve ; these minute ves- 

 sels form a very close irregular 

 hexagonal intermediate net-work, 

 without resolving themselves into 

 branches and twigs like a tree, 

 and so forming a capillary rete. 

 Yet single larger vessels (fig. 

 230, d) proceed from the 

 pulmonary artery to reach some 

 more remote part of the lung. 

 The pulmonary vein, like the pul- 

 monary artery, is partly perforated 

 at every point in its course for the 

 reception of smaller vessels, and 

 is partly formed by larger venous 

 trunks, which collect and bring 

 the blood from greater distances 

 (fig. 230, c). The islets of the 

 thin and indistinctly cellular pa- 

 renchyma, are often of a di- 

 ameter inferior to that of the ves- 

 sels which surround them ; this 

 is the case in the tortoise, for ex- 

 ample (fig. 239), and appears to 

 be the case in man also (figs. 241, 

 242). It is remarkable that 

 even in the more conspicuous 

 branches of the pulmonary vas- 

 cular system the layer of trans- 

 P-nt W in ^mediate con- 



(fig. viii. T. xviii.) tact with the walls or the vessels 



Fig. 245. The greater part 

 of the right lung of a foetal 

 sheep, an inch and a half long, 

 seen under the microscope (af- 

 ter Miiller, De Gland, secern, 

 struct, penit. T. xvii. f. 7). 



Fig 246. Termination of one 

 of the branchings of the bronchi 



