140 PRACTICAL HISTOLOGY. 



Except when the epithelioid cells are first removed, 

 either purposely or accidentally, even a compara- 

 tively long exposure to the action of the nitrate of 

 silver solution will not cause the deeper coats to 

 become stained. This is the case with all structures 

 which are coated with epithelioid cells. 



Preparation 2. Elastic layers ; fenestrated 

 membrane ; muscular tissue. To prepare these 

 several parts, a piece of artery (or vein) is taken (as 

 fresh as possible, but this is not so imperative as for 

 the preparation of the epithelioid layer), and placed 

 for a week or more in a weak solution of bichromate 

 of potash (about 1 in 800), the fluid being changed 

 every other day. The piece is then taken out, pinned 

 down on a cork, with the inner surface uppermost, 

 and a thin strip torn off from the inner surface with 

 fine forceps. This is transferred to a slide, and teased 

 as finely as possible in a drop of water. It will be 

 found advantageous to employ only as much water 

 as will keep the tissue moist, and to add more of 

 this by placing a drop on the cover-glass before it is 

 laid on. If the small pieces are then examined, it 

 will be found that they are for the most part made 

 up of a close network of elastic fibres of varying 

 degrees of fineness. Many of them have very broad 

 fibres and small meshes, so that there may be found 

 in different arteries every transition to the true elastic 

 membrane. This will itself in all probability be met 

 with projecting at the edges of some of the fragments 

 of tissue, or even entirely separated ; the fragments 

 are generally curled at the edges, are often striated, 

 and nearly always exhibit rounded holes. These 

 fenestrated membranes are more frequently met with 

 in the inner coat of the smaller or medium sized 

 arteries (such as the basilar), than in the largest ves- 

 sels (such as the aorta). 



But besides the different kinds of elastic tissue 

 there is also to be found in nearly every such prepa- 

 ration a number of plain muscular fibre-cells scattered 

 about in the fluid; for, in stripping off' the inner 



