188 PRACTICAL HISTOLOGY. 



gastric mucous membrane to five hundred c. c. of a 0.2 per 

 cent, solution of hydrochloric acid. It is then maintained 

 for five or six hours at a temperature of 38 C., when the 

 piece of skin is removed and placed in water for twenty- 

 four hours. Sections may then be cut in any desired 

 direction, stained with logwood, and mounted in glyce- 

 rine ; and although not obtainable very thin, yet, owing 

 to their clearness and transparency, the arrangement of 

 the bloodvessels, and of the little muscles attached to the 

 hairs can be traced with comparative facility. 



Preparation 9. Hairs. To examine a hair, all 

 that is necessary is to place it on a slide in a drop 

 of water, cover with a thin glass, and examine with 

 a moderately high magnifying power. By careful 

 focussing the cuticular scales can often be made out 

 on the surface and at the edges of the hair, especially 

 on the small hairs of the general surface of the body. 

 The medulla is often absent in hair of the head, but 

 may generally be found in those of the beard and 

 whiskers. Many of the black particles which are 

 seen in a hair by reflected light, and especially in 

 the medulla, are merely small globules of air in the 

 interstices of the tissue. That this is so may be 

 proved by cutting off the light which comes from 

 the mirror of the microscope and viewing the object 

 by reflected light, only a moderate power being used. 

 The black particles, if really due to the presence of 

 air, will then appear silvery white, just as in the 

 parallel case of the air which fills the lacunas in a 

 section of hard bone. 



It will be useful to compare the appearances pre- 

 sented by human hair with those exhibited by the 

 hairs of some of the common domestic animals. 

 These are many of them characterized by the regular 

 arrangement of the medulla (this is nearly always 

 present in the hairs of quadrupeds), which forms 

 different patterns in different kinds of animals, so 

 that the species to which the hair belong may often 

 be determined. 



