THE LUNGS. 583 



already somewhat inci'cased thickness of the epitheJium in them, and the 

 rather -wider capillary plexus, to a less extent than in the air-cells. 

 Here, also, wc might recall to mind the dilatation of the bronchial arte- 

 ries and extension of their zone of distribution, in cases Avhere the circu- 

 lation through the pulmonary arteries is interrupted [vide Virchow, in 

 his " Archiv." III., 3, p. 456), in -which cases the bronchial arteries 

 frequently -wholly replace branches of the pulmonary artery, and become 

 respiratory vessels; conditions ^Yhich, from the occurrence of numerous 

 normal anastomoses between the two vascular systems, it is not difficult 

 to explain. 



§ 179. Development of the limgs. — In the Mammalia, the lungs ap- 

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

 anterior wall of the phar?/nx, which are in close apposition, and soon 

 become furnished with 'a common peduncle — the rudiment of the larynx 

 and trachea — and in the composition of -v\'hich the epithelial tube and 

 the fibrous membrane of the intestine take an equal share. 



In the further course of development, there springs from the extre- 

 mities of the original protrusions, a continually increasing 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 ele- 

 ments, the interior epithelium extends itself by spontaneous multiplica- 

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

 time, the fibrous layer surrounding them also grows, and finally consti- 

 tutes the fibrous membrane of the hronchice 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, 0*1G of a line in size, may be recognized, originating in 

 the dilated extremities of the hronchice, which, even at this time, are 

 considerably ramified. As development proceeds and the ramifications 

 of the hronchice 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 0-24-0'48 of a 

 line in size, each of -which, in all probability, is produced 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 lohule, which, -with air-cells of 0-025-0-3 of a line in size, first 

 becomes distinctly visible in the sixth month, althougli, up to the time 

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

 II. 2, p. 323). In the new-born child the secondary lobules measure 

 2-3-4 lines ; the alveoli, before they are filled with air, 0-03, and after 



