THE NERVOUS SYSTEM. 437 



or undergoing any farther decrease in size, their fibres forming 

 loops with closely approximated sides. 



Although, speaking relatively, it is not difficult to make out 

 the structure of the corpus striatum, at all events, in its prin- 

 cipal 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 

 grey substance, on which account they cannot be traced to any 

 great distance. The examination of the grey substance itself, 

 however, is perfectly easy even in these bodies, and its elements 

 — the nerve-cells — present nothing peculiar, except that, in the 

 optic thalami, they are for the most part more deeply coloured, 

 whilst those in the corpora quadrigemina 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 resiiformia, 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, arc 

 not continued into the medullary substance of the hemispheres, 

 because, on the one hand, most of their fibres decrease from 

 the original diameter of 0-0012 — 0001'" clown to the smallest, 

 or less than 0-001 "'; 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 

 hemispheres. The superficial white investment, however, of 

 the ganglia in question, must be excepted, which in any case 

 may effect a relation between them and the hemispheres, as 

 its fibres, measuring 0-001 — 0-003'", 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 



