716 THE RESPIRATORY TRACT. 



entire extent, while other alveoli appear empty. Both these 

 conditions become intelligible if we bear in mind that the blade 

 of the knife passes through some alveoli near their periphery , 

 and through others across their centers ; the former will show 

 the capillaries in front view, in the latter none at all will be 

 visible. The thickening of the capillary reticulum along the 

 walls is due simply to their optical shortening in the top view of 

 the septa. If we look into a basket, the bottom gives the hurdle- 

 work in front view, therefore extended, while the walls show the 

 hurdle condensed, and thus the wall appears thickened. By this 

 simile the structure of the lung becomes easily understood. 



Upon applying the high powers of the microscope, we 

 recognize the broad capillary reticulum at the bottom of the 

 alveoli, while the same reticulum appears shortened and con- 

 densed along the septa. Close above the blood-vessels we see the 

 extremely delicate layer of flat epithelia, which has often the 

 appearance of a granular mass with regularly interspersed nuclei, 

 while the cement-substance is not plainly marked. The injection 

 of a solution of nitrate of silver brings into distinct view the 

 dark brown lines of the cement-substance, interrupted by delicate, 

 transverse light lines, which correspond to the connecting fila- 

 ments (thorns). F. B. Schulze discovered two varieties of lung- 

 epithelia, viz. : small, coarsely granular, and large, pale, nearly 

 homogeneous bodies; this difference is marked only in those 

 human lungs which have been engaged in respiration. Henle 

 maintains that the epithelium cannot be seen in human lungs ; 

 but there is no difficulty whatever in observing it even in 

 injected specimens. (See Fig. 319.) 



Neither lymphatics nor nerves seem to be very numerous in 

 the lungs, and neither the origin of the former nor the termina- 

 tion of the latter are yet explained. Some authors assert that 

 the lymphatics of the lungs are mere clefts in the connective 

 tissue, without any walls of their own. As we know that the 

 lymphatics everywhere constitute a closed system of vessels, with 

 an endothelial lining, contrary assertions, especially regarding 

 the lymphatics of the lungs, unless clearly demonstrated, deserve 

 little attention. The ultimate terminations of the nerves have 

 not yet been discovered. 



Pathologij. In autopsy, the lungs of adults are seldom found in 

 a perfectly normal condition. Besides the hyperaemia which 

 accompanies fatal diseases, serous effusion, called (edema of tlie 

 lungs, is often met with, representing a secondary and concomi- 



