384 SPECIAL HISTOLOGY. 



a plexus, or undergoing any farther decrease in size, their fibres form- 

 ing loops with closely approximated sides. 



Although, speaking relatively, it is not difficult to make out the struc- 

 ture of the corpus striatum, at all events, in its principal features, it is 

 quite otherwise with the optic thalami and corpora quadrigemina, chiefly 

 because the nerve-fibres in these situations are not so much assembled 

 into fasciculi, but are more isolated and most intimately intermixed with 

 the gray substance, on which account they cannot be traced to any 

 great distance. The examination of the gray substance itself, however, 

 is perfectly easy even in these bodies, arid its elements the nerve-cells 

 present nothing peculiar, except that, in the optic thalami, they are for 

 the most part more deeply colored, whilst those in the corpora quadrige- 

 mina are paler. With respect to the nerve-fibres, it is quite certain 

 that the superior portion of the crura cerebri, that is to say, the crura 

 cerebelli ad corpora quadrigemina^ the continuations of the olivary 

 columns, portions of the corpora restiformia, and the eminentice teretes, 

 pass into the ganglia now under consideration, although I have not as 

 yet succeeded in eliciting anything determinate as to the course they 

 take. But I think it may be stated, that the fibrous masses above 

 named, in great part at least, are not continued into the medullary sub- 

 stance of the hemispheres, because, on the one hand, most of their fibres 

 decrease from the original diameter of 0*0012 0'004 of a line down to 

 the smallest, or less than 0-001 of a line ; and on the other, because no 

 such passage of the fibres can be perceived on that side of the optic 

 thalamus, which looks towards the medullary substance of the hemi- 

 spheres. The superficial white investment, however, of the ganglia in 

 question, must be excepted, which in any case may eifect a relation 

 between them and the hemispheres, as its fibres, measuring 0'001-0'003 

 of a line, or even more, disposed in fasciculi, and crossing each other 

 ' horizontally in various directions, do not appear to terminate in it. 

 Neither is the relation of the optic thalami to the corpora quadrigemina, 

 and that of the fornix to the latter, by any means clear, so that it is 

 pleasing, at all events, to find that another important question admits of 

 a more satisfactory solution. When the external portion of the optic 

 thalamus is examined, it will be found that it adjoins a considerable mass of 

 white substance, which at first sight appears to be a continuation of the 

 basis of the cerebral peduncles passing below and external to the optic 

 thalamus, between the lenticular and caudate nuclei of the corpus stria- 

 turn, to enter directly into the medullary substance of the hemispheres. 

 Closer observation renders it obvious, that this white substance, as has 

 been said before, in part enters the corpus striatum, particularly the 

 lenticular nucleus, and in part radiates from without inwards, from the 

 hemisphere into the optic thalamus. That is to say, very numerous 

 white fasciculi, visible even to the naked eye, coming from the hemi- 



