394 Pierre A. Fish 



slightlj'- greater density of the fluid is believed to be more ad- 

 vantageous than otherwise, since it buoys the brain until the 

 tissue has begun to harden and can partially support its own 

 weight. The pressure is nearly enough equal on all sides to 

 prevent any noticeable change of form. It is recommended that 

 the cavities of the brain be filled with the mixture (coelin- 

 jected) and if practicable the blood-vessels also injected. 

 After an immersion of about three days the specimen should 

 be transferred to equal parts of the foregoing mixture and 

 seventy per cent, alcohol for a week or more, where on account 

 of the lesser specific gravity it should rest upon a bed of ab- 

 sorbent cotton ; it is finally stored in 90% alcohol. 



The addition of the zinc chlorid to the solution is to expe- 

 dite the hardening, to differentiate the tissue, and to insure 

 a more equable and penetrating action. Osier attributes 

 the diflFerential efi^ect to the glycerin or some impurity in it. 

 Experiment has not confirmed his statement. Zinc chlorid 

 coagulates the blood and renders it much darker than usual. 

 The highly vascular condition of the cinerea would soon ren- 

 der it susceptible to the action of this salt, and it would in 

 general assume a .shade relatively much darker than the alba. 

 The sodium chlorid is supposed to render the zinc more solu- 

 ble, and to some extent to lessen its causticity. The glycerin 

 is also useful in this latter respect, but its chief use besides 

 preservation is to bring the fluid up to the required specific 

 gravity. 



A one-fifth per cent, solution of picric acid in fifty per 

 cent, alcohol has been used by Professor S. H. Gage with 

 very successful results upon a human brain. The specimen 

 was carried up gradually to 95% through the intermediate 

 grades of alcohol. He has also obtained excellent prepara- 

 tions of fetal brains by injecting the preservative through a 

 hypodermic needle into the brain cavities. 



"Dry" preparations are those which may remain perman- 

 ently exposed to the atmosphere at the ordinary temperature, 

 without apparent detriment. There are essentially two 

 methods of preparation, the one consisting of actual dessication 

 or mummification, in which the specimens remain hard and 



