114: LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



The stratum lemnisci, then, contains two elements, the 

 upper and the lower fillets. The lower (better, mid-brain) fillet 

 arises chiefly from a system of fibres which has not been men- 

 tioned before, the deep marrow of the roof of the mid-brain ; but 

 it also arises in part from the ganglion of the posterior quadri- 

 geminal body. This is clearly shown in an oblique section down- 

 ward through both pairs of quadrigcminal bodies, as represented 

 in Fig. 65. The ganglion above mentioned consists of a large, 

 roundish nucleus permeated by a net-work of fine fibres. This 

 is the only ganglion situated in the posterior quadrigeminal 

 body ; the latter, therefore, does not present that stratified ap- 

 pearance, alternately gray and white, which is characteristic of 

 the anterior quadrigeminal body, the ganglion opticum. It is 

 connected with its fellow of the opposite side by the fibres run- 

 ning over the aqueduct of Sylvius. 



Phylogenetically the deep marrow is a very old system. 

 It is present even in the simple brains of the lower vertebrates, 

 and in these, as in the human being, early receives its medullary 

 sheaths. Its fibres arise in the roof of the mid-brain, springing 

 from layer^ which lie beneath the origin of the optic nerve. 

 From this point they at first radiate inward, but, on reaching 

 the central gray matter, they turn and run ventrad. The most 

 lateral of these fibres, united with those which come from the 

 opposite side, pass into the lemniscus ; the more median fibres, 

 however, pass down around the aqueduct of Sylvius and decus- 

 sate with the analogous fibres of the opposite side. In human 

 beings the termination of these fibres has not yet been made 

 clear. We call them by the name proposed by Forel, the 

 " fountain-like" decussation of the tegmentum (Figs. 66, 70, 72). 

 In fishes and birds these particular fibres of the deep marrow 

 are so extensively developed that their course is much more 

 readily recognizable. In them, as well as in amphibians and 

 reptiles, we see that, except so far as they pass into the fillet, 

 this system of fibres is contained Avholly in the mid-brain and 

 terminate in cells situated both on the same and on the opposite 



