178 SPECIAL HISTOLOGY. 



respiratory vessels ; conditions which, from the occurrence of 

 numerous normal anastomoses between the two vascular systems, 

 it is not difficult to explain. 



§ 179. 



Development of the lungs. — In the Mammalia, the lungs ap- 

 pear a little after the liver, in the form of two hollow protru- 

 sions of the anterior wall of the pharynx, which are in close 

 apposition and soon become furnished with a common peduncle 

 — the rudiment of the larynx and trachea — and in the com- 

 position of which the epithelial tube and the fibrous membrane 

 of the intestine take an equal share. 



In the further course of development, there springs from 

 the extremities of the original protrusions, a continually increas- 

 ing number of arborescent processes, which differ entirely from 

 what may be observed in most other glands. From their first 

 formation they are always hollow, and in the sixth month the 

 air-cells are developed from their invariably clavate, dilated 

 extremities. During this growth of the glandular elements, the 

 interior epithelium extends itself by spontaneous multiplication 

 of its cylindrical cells (probably by division), whilst at the same 

 time the fibrous layer surrounding them also grows, and finally 

 constitutes the fibrous membrane of the bronchia and air- 

 cells, together with the vessels and nerves. In the second 

 month, in the human embryo, the large pulmonary lobes are 

 already formed and besides them, smaller divisions also, 016'" 

 in size, may be recognised, originating in the dilated extremities 

 of the bronchia, which, even at this time, are considerably 

 ramified. As development proceeds and the ramifications 

 of the bronchia are multiplied, these gland-granules, as I have 

 termed them, become more and more numerous, and ultimately, 

 in the fifth month, are aggregated so as to form smaller lobules, 

 of 0*24 — 048"' in size, each of which, in all probability, is pro- 

 duced from a single gland-granule, or bronchial termination of 

 the second month. Each of the gland-granules of these lobules, 

 which correspond with the secondary lobules of the future lung, 

 by continued budding, finally constitutes a primary lobule, 

 which, with air-cells of 0*025 — 0-03'" in size, first becomes dis- 

 tinctly visible in the sixth month, although, up to the time of 

 birth, new alveoli are constantly superadded [vide Mikr. Anat. 



