700 THE MICROSCOPE. 



regular manner throiigli tliem ; and ^vllereve^ tliere is 

 reason to suspect the existence of disease, small portions 

 should be removed, and examined while perfectly fresh 

 under high magnifying powers. The nature of the lesion 

 having been thus ascertained, the morbid parts, with 

 some of the surrounding healthy tissue, should be re- 

 moved, and after being divided, if necessary, into smaller 

 portions, should be macerated in a weak solution of chromic 

 acid. It is very important to cut and subdivide the 

 parts, when necessary, in such a manner, that their rela- 

 tion to the rest may be recognised after they have become 

 hardened for the purpose of making sections. Thus, — 

 unless the locality of the lesion require a different course, 

 — one incision may be made through the crura cerebri, 

 immediately in front of the small or motor roof of the 

 trifacial nerves, and a third through the base of the 

 anterior pyramids, immediately below the attachment of 

 the sixth pair of cerebral nerves. With regard to the 

 «pinal cord, if it be cut up into small portions and 

 hardened in chromic acid, it will be found better to divide 

 it in three places : 1st, through the middle of the cervical 

 enlargements ; 2d, through the middle of the dorsal 

 region ; and 3rd, through the middle of the lumbar 

 enlargement. 



If, however, the lesion be suspected to exist at another 

 point, the cord must be divided there ; as it is highly 

 important that the nature of any morbid portion that 

 may be found should be examined under the microscope 

 in a 'perfectly fresh state ; for the nerve-fibres undergo a 

 considerable and very deceptive alteration by the action 

 of chromic acid. But it is no less important, and indeed is 

 absolutely necessar}^ for an exact and complete investiga- 

 tion, that entire portions of the cord, in the locality of the 

 lesion, be hardened in chromic acid, so that thin, but 

 ■perfect sections may be made for examination under the 

 niicroscope. 



" The strength of the chromic acid solution should differ 

 for different parts of the cerebro-spinal centres. Foi' the 

 convolutions of the cerebral hemispheres and cerebellum, 

 the proportions should be one of the crystallised acid to 

 about four hundred of water, while for the pons varolii, 



