ORGANS OF THE BODY. 



457 



The lymphatic network is very dense, and may be divided into two 

 layers separated from one another by fibrous tissue. The superficial 

 canals traverse the interstices of a reticulated layer of connective-tissue, 

 the substratum of the serosa. Here their walls, composed of vascular 

 cells, are covered solely by the epithelium of the membrane between 

 whose cells those orifices already described in section 208, fig. 381, 2, 3, 

 are situated. 



Absorption from the cavity of the pleura is effected through the 

 agency of the respiratory movements in the intercostal spaces, and the 

 varying amount of tension to which the connective-tissue in which these 

 canals are situated is subjected thereby. The contents of the networks 

 formed by the latter are received by valved vessels running along tho 

 ribs towards the vertebral column and by the mammary twigs. 



243. 



Turning now to the composition of the pulmonary tissue, we find that 

 only of the products of decomposition occurring in the fluids with which 

 it is saturated is anything reallv known. Cloetta obtained inosite, 

 taurin, and leucin from the 

 lung of the ox. The human 

 lungs, also, were found to con- 

 tain leucin in considerable 

 quantity. In the foetus the 

 organ yields glycogen (Ber- 

 nard, Rouget). 



The development of tho 

 lungs (fig. 439, 1) takes place 

 very early in the same way as 

 the large glands connected 

 with the intestinal tube, 

 namely, in the form of two 

 hollow processes (c) attached 

 by one stalk (a) to the anterior 

 wall of the pharynx. This 

 body is hollow from the very 

 commencement. Both the 

 internal and middle germinal 

 plate are here represented, the 

 former in the cellular layer 

 (c), the latter in the fibrous 

 wall of the part (&). From 

 the cellular layer the epithe- 

 lium of the respiratory tract 

 is derived, while in the ex- 

 ternal investing mass we 

 have the rudiments of all the 

 fibrous and cartilaginous por- 

 tions of the air- passages, 



bronchi, and lungs. From these blind tubes of the glandular plate an 

 ever-increasing number of new sacculi (d) are now given off into the 

 surrounding substance by means of cell-multiplication, so that the arbores- 

 cent arrangement of the respiratory canals becomes more and more marked, 

 as the enveloping layer decreases progressively in proportion. At the 



Fig. 439. Development of the lungs. 1. Plan of the for- 

 mation of the whole organ, o, common canal (the future 

 trachea) dividing into (c) the two bronchi, with their 

 incipient bud-like saccules, (<f); 6, the surrounding 

 fibrous mass. 2. Ramifications farther advanced, from 

 the lung of a human faetus about four months old. a, 

 the tube; 6, lobulated dilatations lined with cylinder 

 epithelium, from which the infundibulaare formed appa- 



rently 3. The same strongly magnified, a, cylinder 

 epithelium; c, cavit 

 remainder of &, fig. 



epithelium; c, cavity; 6, the investing fibrous layer, the 

 1. 



