ORGANS OF THE BODY. 



455 



bably only receive the blood returning from the thick walls of the larger 

 bronchi, and from the lymphatic glands and pleura around the root of 

 the organ. The finer and internal venous radicals, on the other hand, 

 coming from the smaller divisions of the air passages, and which corre- 

 spond to the distribution of the bronchial arteries, empty themselves into 

 the branches of the pulmonary veins. 



Lymphatics, as has long been known, are present in the lungs in con- 

 siderable number. They may be divided into two classes : into super- 

 ficial (arranged in retiform interlacements immediately under the serous 

 covering of the organ) ; and into deep, which may be traced outwards 

 along the air passages into the bronchial glands. Both of these sets of 

 vessels communicate freely, however, with one another. 



Not long since Wywodzoff was fortunate enough to succeed in injecting 

 the radicals of the lymphatics in the walls of the alveoli in the lungs of 

 the dog and horse, and Solwrsky also in the first named animal and in 

 the cat. In these walls are found lacunse, which are enlarged opposite 

 the meshes of the capillaries. They 

 cross the capillaries, without, how- 

 ever, forming sheaths of any kind 

 around them. Soon after, however, 

 the lymphatic canals as they pass 

 away commence to occupy the adven- 

 titia of the blood-vessels. 



We now come to the considera- 

 tion of the epithelium of the air- 

 cells still a subject of controversy, 

 and which, has been recently the 

 object of the most earnest investiga- 

 tion. 



Turning then, in the first place, to 

 the lung of the frog, we find the 

 arrangement of parts of the simplest 



kind (fig. 436). The whole respiratory portion of the organ is lined 

 with a single continuous layer of flattened nucleated epithelial cells. 



But the lungs of the mammalia and man present greater difficulties. 



Here we must first study the structure of the parts at an early period 

 of existence, if we would understand 

 it in the adult body. 



In the mammal foetus we likewise 

 find a continuous epithelium lining 

 both pulmonary vesicles and alveolar 

 passages, and entirely the same in 

 both. Its elements are flat polyhed- 

 ral cells, with nucleus and protoplasm. 



After birth, however, several 

 changes become rapidly apparent, 

 consequent upon the commencement 

 of respiration. Only a small portion 

 of the epithelium preserves its former 

 character. Over the projections of 

 the capillaries, and all other promin- 

 ences, we can find much larger pale cells without protoplasm or nucleus 

 in many cases. 



30 



Fig. 436. A portion of an air-cell from the 

 lung of a frog. 



Fig. 437. 



