THE RESPIRATORY TRACT. 259 



bronchial tubes of cats and dogs while the animals were liv- 

 ing, and found, post-mortem, that the carmine had penetrated 

 through the interepithelial cells above described into the lym- 

 phatic vessels below. 



The interepithelial cells have a small nucleus which stains 

 more deeply with hsematoxylon than do the nuclei of the ordi- 

 nary epithelial cells. In vertical section they are more opaque 

 than the epithelia, and, when seen on the surface of the mem- 

 brane, appear as dark spots among the ciliated cells. 



The lymphatic capillaries join to form larger trunks which 

 run along the sides of the bronchi communicating freely with 

 each other and with those of the neighboring blood-vessels. 

 They are called by Klein the peribroncMal lymphatics. 



Beneath the mucosa, and between it and the mucous glands, 

 are numerous bundles of yellow elastic tissue having a longi- 

 tudinal direction. Some of the bands are quite thick, particu- 

 larly in the posterior wall, and raise the mucous membrane in 

 longitudinal folds. 



The mucous membrane of the trachea and bronchi has a 

 rich network of capillaries. The racemose glands are also sup- 

 plied with a vascular network which ramifies in the fibrous tis- 

 sue by which they are surrounded. The natural injection of 

 these vessels, which occurs in cases of bronchitis in the human 

 subject, is often sufficient for their examination. 



The mode of termination of the nerves has not been ascer- 

 tained. 



The trachea should be hardened in chromic acid or Miillers 

 fluid, followed by alcohol. The sections may be stained in 

 hsematoxylon. In order to preserve the ciliated epithelium, it 

 is well, as Professor Rutherford suggests, to cut the sections 

 with the freezing microtome. The lymphatics can be injected 

 by puncture. 



The smaller bronchi and lungs. Beyond the primary bron- 

 chi (or first division of the trachea) the muscular fibres encircle 

 the tubes inside of the cartilaginous and fibrous layer ; indeed, 

 the primary divisions show the first sign of this new arrange- 

 ment. The cartilages change from incomplete rings to irregu- 

 larly shaped plates, which are found on all sides of the tubes, 

 but their microscopic structure remains unaltered. The longi- 

 tudinal elastic fibres are contained between the muscular and 

 mucous coats. The tubes divide and subdivide generally in a 



