132 



iiess or absence of medullary sheath, yet capable of being 

 well demonstrated when their torn ends project from the 

 edge of a section. 



Lockhart Clarke in part attributes the decreasing diameter 

 of the vertical bundles of fibres to some of the fibres turning 

 round in the inner layer so as to become horizontal; but this 

 is really not the state of matters which exists. The examina- 

 tion of extremely thin sections, which have been preserved 

 for only a few days in weak chromic acid, or in a filtered 

 mixture of oxgall and S23irit, shows the vertical and hori- 

 zontal fibres crossing one another with the utmost clearness 

 and without the slighest tendency of the one set to pass into 

 the other. Unfortunately, such specimens cannot be pre- 

 served, but they are very distinct while they last. The 

 horizontal and vertical fibres in some instances communicate, 

 but it is through the medium of nerve-corpuscles. 



The typical, and by far most frequent, form of nerve-cor- 

 puscle found in the convolutions of the brain is the pyra- 

 midal ; it presents the same structure, both in the case of the 

 large and that of the small corpuscles, and has been described 

 by the diiferent recent writers on the subject. The apex of 

 the pyramid is always directed towards the surface, and is 

 prolonged into a nerve fibre, Avhich passes right on to join — 

 probably always — the bundles of horizontal fibres at and 

 near the surface. I say that, probably always it does so, for 

 although I have only once or twice traced such fibres 

 to the surface, and seen the turning round, yet one may 

 argue from the constantly vertical course which they are 

 seen to pursue when traced for considerable distances from 

 their origins in the deep strata. Arndt describes- instances 

 in which he made sure that this main process, or apex fibre, 

 of the nerve-corpuscle was continued into a dark-bordered 

 nerve fibre ; and I am disposed to go further, and to say that 

 for a short distance, at least, it is always dark-bordered. In 

 the case of the larger corpuscles, the granular contents of the 

 corpuscle are continued for some distance within the apex 

 fibre. 



Lockhart Clarke, as well as others, describes the apex 

 fibre as " giving off minute branches in its course ;" Arndt 

 describes it as unbranching, except in the cornu ammonis. 

 I believe that Arndt is right in considering that in the 

 ordinary arrangement this fibre does not branch. I have 

 certainly seen, as he also has done, an appearance of one or 

 two minute branches given off laterally from it, while the main 

 fibre continues its uninterrupted course ; but such appear- 

 ances seemed to be due to the accidental adhesion of portions 



