THE LARGER BLOODVESSELS. 139 



more may be taken from other parts. In making 

 sections when the piece is held in this way, it will 

 be found convenient to cut from the operator. 



The slices are taken up on a needle or section-lifter 

 and placed on a slide with the stained surface upper- 

 most ; they should be in as little spirit as possible, 

 but at the same time should not be allowed to be- 

 come actually dry. A drop of the ordinary glycerine 

 solution is placed on a cover-glass, and this is quickly 

 inverted over the sections. On examinig them with 

 the microscope, the outlines of the epithelioid cells, 

 and perhaps also their nuclei, will be seen in those 

 sections which were made from the gray part of the 

 bloodvessel. In sections, however, which include 

 any of the patches which look brown to the naked 

 eye, it will be found that the difference of color is 

 due to the epithelioid cells having at these parts 

 become accidentally rubbed or washed away before 

 the silver solution was allowed to act ; for since the 

 subjacent tissues (the sub-epithelial connective tissue, 

 if present, arid the muscular tissue of the middle 

 coat) contain more intercellular or ground-substance 

 than the epithelioid layer (where it only occurs in 

 tine lines between the cells), they assume a browner 

 appearance after the reduction of the silver, and 

 show under the microscope in the one "case irregular 

 white patches the cell spaces upon the brown 

 ground, and in the other transversely arranged 

 lanceolate white markings the plain muscular fibre 

 cells with a variable amount of ground-substance 

 between. The latter appearance may be obtained all 

 over the preparation if the bloodvessel which is to 

 be treated with silver nitrate is first brushed with 

 a camel-hair pencil moistened with distilled water, 

 for by this means the epithelioid cells are removed, 

 and also, for the most part, the sub-epithelial con- 

 nective tissue Avhere present, the elastic layer being 

 the only part of the internal coat which remains, 

 and since this does not reduce the silver salt the 

 muscular laver is the one which is seen in such cases. 



