152 PRACTICAL HISTOLOGY. 



water and strong spirit, freshly prepared), and expos- 

 ing it to the light in this for an hour or more. The 

 cutting off of the intestine must be performed in the 

 same fluid and the mesentery floated from it on to 

 the slide. The method has the advantage not only 

 of effecting the reduction of the metal with greater 

 surety, but also of rendering it easy to obtain the 

 membrane free from creases, for the mesentery is 

 partly hardened by the spirit while in a state of 

 extension, and continues in this condition when 

 floated on to the slide, so*that it is seldom necessary 

 further to extend it by artificial stretching. In 

 these silvered preparations little but the epithelioid 

 cells can be made out, for the rest of the tissue 

 generally remains almost unstained, and becomes 

 very transparent in glycerine. 



Preparation 5. To exhibit the muscular struc- 

 ture of the small arteries and veins, and the nuclei 

 of their epithelioid lining and of the walls of the 

 capillaries, the vessels are stained with logwood. 

 This is done by immersing the mesentery or other 

 vascular membrane, either fresh, or better after hav- 

 ing lain for a day or two in very weak bichromate 

 of potash solution, in a dilute solution of logwood 

 alum, until distinctly colored; then place the tissue 

 in water, and mount it, with the same precautions 

 as before to prevent creasing, in glycerine. For the 

 structure of the small arteries the pia mater from 

 the human brain may be used. A small piece is 

 stripped off with forceps ; and as it consists almost 

 entirely of small arteries and veins, and moreover 

 a few capillaries are generally dragged out with, it 

 from the cerebral substance, the structure of all the 

 vessels is, after staining, very well displayed. The 

 small veins are here exceptional in being entirely 

 devoid of a muscular coat, whereas the arteries have 

 this coat well developed, and it is particularly well 

 shown in consequence of the staining of the trans- 

 verse nuclei of the muscular fibres by the logwood. 

 Within these Tnay be detected, by carefully using 



