456 NERVOUS SYSTEM— GENERAL METHODS 



weight during the process. The spinal cord or small portions 

 of any region of the encephalon may be cut into thin slices, laid 

 out on cotton or glass wool in a vessel into which the hardening 

 fluid is poured. The specimens may also be suspended in 

 the liquid. Another good plan consists in adding to the 

 hardening fluid just enough glycerin or sodium chloride to make 

 tissues float. 



If several pieces are placed in the same vessel, they should 

 never be put on top of each other. Voluminous organs to be 

 hardened in toto should be at least incised as deeply as possible 

 in the less important regions. With the exception of the dura 

 mater, the membranes are not generally removed at first, as 

 they serve to protect the tissues. They can be removed partially 

 or entirely later on when the hardening has made some progress. 

 In the case of material intended for Golgi's methods it is best not 

 to remove them at all. 



The spinal cord, medulla oblongata and pons Varolii may be 

 hardened in toto, and the preparation hung up in a cylindrical 

 vessel with a weight attached to its lower end to prevent it from 

 becoming distorted. The dura should be slit up along the front 

 and back of the cord and each half cut across in two or three 

 places to prevent longitudinal shrinkage. 



A useful method of treating a brain which is to be studied from 

 an anatomical standpoint is to make sagittal incisions through 

 the corpus callosum and infundibular recess, allowing the cere- 

 brospinal fluid to drain away, and then to suspend the brain in 

 4 or 5 litres of formol saline by a string, either passed between 

 the basilar artery and the pons, or passed through the cerebral 

 hemispheres from side to side. 



The action of most hardening fluids is greatly enhanced by heat. 

 But in the judgment of most histologists this rapid hardening 

 is not, as a rule, attended by good results, and one should have 

 recourse to it only for particular reasons and special purposes 

 after a tentative experiment at establishing the degree of tempera- 

 ture at which the desired results may be obtained without injuring 

 the delicate structure of the nerve-tissue. 



On the other hand, the hardening action at room temperature 

 of certain reagents, such as solutions of chromic salts, proceeds 

 so slowly that decomposition may set in before the fluid has 

 had time to act effectively. For this reason voluminous pre- 

 parations which are to be hardened in toto in solutions of chromic 

 salts, and were not injected as described in § 978, should be 

 put away in a very cool place or in an ice-chest. A human 

 cerebral hemisphere may require eight or nine months for harden- 

 ing in this way. 



The volume of the fluid should always be very large in pro- 

 portion to that of the pieces of tissue and to their number. 



