186 PRACTICAL MICROSCOPY. 



and consists of two lateral masses and a connecting link or commis- 

 sure. Near the central portion of the figure, a small circular opening 

 presents the transversely divided central canal. This is in commu- 

 nication, in the medulla, with the fourth ventricle, and will serve as 

 a starting-point for our study. 



The gray matter completely surrounds the central canal, and its 

 outline resembles the capital H. The bars or columns each present 

 anteriorly a blunted extremity, horn or cornu, while the posterior 

 cornua are pointed. The lateral bars or columns are connected, as 

 we have seen, a portion of the connecting substance passing in front 

 and a portion behind the central canal the anterior and posterior 

 gray commissural lands. 



The white substance is divided anteriorly by the anterior median 

 jissure, which sections the cord nearly, but not entirely, to the ante- 

 rior gray commissure. A corresponding division appears posteriorly 

 (the posterior median -fissure) which does not divide the cord posteri- 

 orly as completely as does the previously named fissure anteriorly; 

 but the divison is indicated by a band of neuroglia, which penetrates 

 entirely to the posterior gray commissure. The two masses of white 

 substance thus indicated by more or less complete central division are 

 termed lateral white columns, and these are united just in front of 

 the anterior gray commissure by white nerve tissue the white com- 

 missure. The spinal nerves take, origin from the gray cornua, the 

 anterior roots from the anterior and the posterior roots from the pos- 

 terior cornua. The white substance consists essentially of medullated 

 fibres which, with the exception of the anterior spiral nerve roots and 

 the commissural fibres, pass mainly in a longitudinal direction. 



PRACTICAL DEMONSTRATION. 



Nerve tissue should, under all circumstances, be hardened in 

 Miiller's fluid. The cord should be obtained as nearly fresh and un- 

 injured as possible; cut transversely with a sharp razor into pieces 

 half an inch long, and placed immediately in the fluid in the pro- 

 portion of a pint of the mixture to two ounces of tissue The solution 

 should be thrown away after twenty-four hours, and a fresh supply 

 provided. It should be again changed after three days, and again after 

 another week. After four weeks the bichromate should be poured off, 

 and the tissue rinsed once with water; after which the hardening is to 

 be completed with alcohol in the ordinary manner, i. e., commencing 

 with the weak spirit. 



After hardening, pieces from the different regions should be cut, 

 and this is best effected by the infiltration methods. Transverse 

 .sections are the most instructive, although the student should after- 

 ward study longitudinal cuts. The sections must be thin, but not 



