LUNGS 321 



tissue, the walls of the vessels as well as the respiratory cells. That 

 either the walls of the respiratory epithelium or the walls of the blood 

 vessels are not dispensed with, and one wall used to separate the two me- 

 dia, is possibly due to a number of reasons, among which can be brought 

 to mind a lack of specialization in the cooperation of the two tissues. 

 This may not have occurred because of the lack of any actual need of it 

 or because of an inherent impossibility of an ectodermal epithelium 

 becoming the wall of a blood channel, or of a connective tissue being 

 situated on an external surface of the body. In some forms it is very 

 hard or even impossible to detect the presence of this blood-channel wall. 



The walls of the blood vessels, where they are in contact with a spe- 

 cialized respiratory epithelium, are as thin as possible, not more than one 

 cell in thickness. This single layer is sometimes so thin that it can be 

 seen with the greatest difficulty. In some cases it is clear that it does 

 not exist, thus showing an exception. 



The accessory tissues found in connection with the specific cells of 

 respiration are but few in number. A small amount of connective 

 tissue and muscle with a very small nerve supply are all that are directly 

 concerned. The muscles, cartilages, and other tissues of the breathing 

 passage and gills will not be considered here, as their functions are but 

 indirectly related. 



Technic. These tissues may be treated as are any other delicate 

 epithelia. When well hardened, they can be seen in section and dis- 

 tinguished from the underlying connective tissues by their hard outlines 

 and compact texture, as well as by the peculiar transparent appearance 

 that has already been commented upon. Teasing gives no important 

 information, and is best put aside for the use of nitrate of silver on the 

 fresh tissue. By staining the cement substances in this way, the boun- 

 daries of the cells are brought out and their relations with one another 

 clearly demonstrated. Lungs should be gently distended as in life at 

 the time of fixation, in order that the elements may present a natural 

 appearance. Over distention is more harmful than the reverse. 



LITERATURE 

 See the general text-books. 



AIR-BREATHING RESPIRATORY TISSUES 



The respiratory cells of the salamander are found on the inner surface 

 of the lung, which is an invagination of the pharynx, and is provided 

 with a supply of fresh air by the breathing of the animal. These cells line 

 the entire inner surface of the lung and rest on a connective tissue with 

 no well-defined basement membrane. Each one does not touch this 



