64 yournal of Cojuparative Neurology ajid Psychology. 



medial than in the teleosts, so that all the real lemniscus fibers 

 run to one side of it. (4) A small number of fibers of this nucleus 

 run caudad, of which I shall speak later. 



I shall next describe the course of the principal mass of the deep 

 layer of the tectum, whose fibers do not form a part of the com. 

 posterior, but just as in the teleosts form the lejnnisciis, or fillet, 

 which, together with the thalamo-spinal tracts, in the selachians 

 fills nearly the whole central mass of the mid-brain (see Fig. 7, 

 Plate XIV). The division of the lemniscus fibers into four parts, 

 as in the teleosts, is not so easy in the selachians. The resem- 

 blance, however, is easily seen to this extent that part of the fibers 

 decussate and part take an uncrossed course. One gets the im- 

 pression that the more medial fibers of this system decussate, while 

 the more lateral fibers generally take the direct course and con- 

 tinue to be more lateral in the oblongata also, while in this part 

 of the brain the crossed fibers lie, as in the bony fishes, nearer the 

 ventral surface and the raphe. It is, however, extremely difficult 

 to distinguish in the oblongata the three groups which are so plain 

 in the teleosts. In the latter fishes, moreover, a part of the 

 lemniscus took its origin from the nucleus lateralis mesencephali 

 in the colliculus. 



We found, as already mentioned, a fiber tract originating in the 

 colliculus of the selachians and running; backward and minglinp: 

 with the fibers of the thalamo-spinal and tecto-spinal bundles. 

 Naturally it is quite impossible to say whether these fibers repre- 

 sent a colliculus portion of the lemniscus or whether they represent 

 the fasciculus longitudinalis lateralis. The latter tract differs 

 fundamentally from the lemniscus (though in human anatomy 

 very often included with it) in that it is not direct like a part of the 

 lemniscus, nor crossed in the com. ansulata like the other part, 

 but it passes on to the acusticum region and constitutes the greater 

 part of the fibrae arcuatae dorsales of that region. It is quite 

 impossible to say whether the caudal attachment of the colliculus 

 in the selachians is lemniscus or lateral tract; but this can be 

 regarded as sure that in the selachians, whose acoustic centers 

 and lemniscus fibers are so strongly developed, the colliculus can 

 represent only a small part of that structure in the teleosts, and we 

 must surely assume that the greater number of fibers which in the 

 teleosts arise from (or more properly terminate in) the colliculus 

 end in the selachians in those layers of the mid-brain from which 



