142 HOW TO WORK 



fully its nature in the different members of the creation, and espe- 

 cially in the lowest forms in which its existence has been proved, 

 for there we may be sure to find it in its simplest condition, and the 

 mind will be better able to appreciate the exact meaning of the 

 structures which are superadded, and the more elaborate anatomical 

 detail which is met with in the higher animals, than if we commenced 

 our researches upon the most perfect examples of the structure. 



In the examination of the mammalian kidney, the epithelium and 

 fragments of the tubes may be readily obtained by scraping the 

 freshly cut surface. In this manner also Malpighian tufts may often 

 be separated, but it is impossible to ascertain the relation of the 

 different structures to each other, as by the process of scraping they 

 are inevitably very much torn. A thin section in which these points 

 may be demonstrated, is obtained either with a sharp thin-bla-ded 

 knife, or more advantageously with a Valentin's knife, by which 

 means a section including both the cortical and medullary portion of 

 the organ may be >made. After washing the section very slightly, it 

 may be placed with a drop of water between two pieces of glass, and 

 examined in the microscope, first using a low power (an inch glass), 

 by which the general arrangement of the tubes will be seen, and 

 afterwards a quarter of an inch object-glass, by the aid of which the 

 different characters of the epithelium in the straight and convoluted 

 portions of the tubes may be demonstrated. 



242. Basement Membrane, Matrix, and Vessels. Just at the 

 edge of the specimen, a portion of a tube stripped of epithelium, and 

 exhibiting the basement membrane very distinctly, may often be 

 observed. 



The appearance which has been described as resisting from the 

 presence of a matrix may be seen very clearly in a section of the 

 kidney of a mouse, or in that of many other rodents. It must, how- 

 ever, not be forgotten how very difficult it is to say how much of the 

 appearance is due to the presence of the walls of the tubes and 

 capillary vessels, and how much to the existence of a structure (the 

 so-called matrix) independent of, and occupying the intervals between, 

 these. Where the capillaries are injected with transparent injection, 

 no fibrous appearance is to be detected; and I believe, at least in 

 healthy kidneys, that the material resembling fibrous tissue, really 

 consists of the walls of .the tubes and the shrunken and otherwise 

 altered capillaries. 



Here and there, apparently upon the vessels of the Malpighian 

 tuft, a few cell-like bodies are often seen. These have been described 

 by some as epithelial cells upon the external surface of the vessel, 

 but the researches of Mr. Bowman proved that the vessels are quite 



