39° DEVELOPMENT OF THE LUNGS. [sect. 179. 



more especially the muscular and fibrous coats of these canals. As may be 

 easily understood, the two systems of vessels have here a certain connection 

 between each other, aud, accordingly, the older anatomists, as Holler, Som- 

 mering, and Reisseisen, are quite right when they speak of a connection 

 between the two systems of vessels. According to Adriani and Rossignol, 

 the bronchial arteries and veins can be injected from the pulmonary veins, 

 and vice versa, the pulmonary veins from the bronchial arteries, although the 

 bronchial vessels cannot be injected from the pulmonary arteries. 



Supported by these facts, we are j ustified in ascribing to the finest bronchia 

 a share in the process of exchange of gases in respiration, although, on 

 account of the somewhat greater thickness of the epithelium, and the some- 

 what wider capillary network of the fine air-tubes, their share must be inferior 

 to that of the pulmonary vesicles. We may also make mention here of the 

 enlargement of the bronchial arteries, and of the extension of their region 

 of distribution in cases of disturbance of the circulation in the pulmonary 

 arteries (compare Virchow, in his Archiv., iii. 3, p. 456). Here the bronchial 

 arteries frequently replace whole branches of the pulmonary arteries, and 

 become actual respiratory vessels ; conditions which are readily explained by 

 the occurrence of numerous normal anastomoses between the two systems of 

 vessels. Very recently, Beau has maintained that the pulmonary arteries 

 supply the mucous membranes i of all the bronchial tubes, even up to the 

 trachea. 



§ 179. Development of the Lungs. — In mammalia, the lungs 

 appear somewhat after the liver, as two hollow protrusions of the 

 anterior pharyngeal wall, situated close to each other. They soon 

 become furnished with a common pedicle — the rudiment of the 

 larynx and trachea — in the construction of which the epithelial 

 lining and the fibrous coat of the pharynx are equally concerned. 

 Developing further, the terminations of the two primitive pro- 

 trusions give out a number of processes, which constantly increase 

 by ramification, and differ from those observed in most other 

 glands, in being always hollow from their commencement onwards. 

 In the sixth month, the pulmonary vesicles develop from the 

 dilated, club-shaped extremities of these processes. During the 

 growth of the gland-elements, the inner epithelial layer extends 

 itself by independent multiplication of its cylindrical cells (probably 

 by a process of partition) ; whilst at the same time, the surrounding 

 fibrous layer continues also to grow independently, and this layer 

 eventually gives rise to the fibrous coats of the bronchia and air- 

 cells, together with the vessels and nerves. In the human embryo, 

 the large pulmonary lobules are formed in the second month, and 

 besides these, smaller segments may be distinguished (o - i6'" in 

 size), which arise from the extremities of bronchia, after these have 

 become already considerably ramified. To these segments I have 

 given the name of gland-granules (in Ecker, Icon. Phys., tab. x., 



