Herrick, Brain of Certain Reptiles. 133 



from the deeper portions of the tectum and run parallel to the 

 optic tracts cephalad and penetrate the geniculata. The caudal 

 brachium is difficult to make out certainly but seems to run par- 

 allel with the caudal optic tract and to be associated with the 

 infracommissure tract, thence with the cerebrum. It is ob- 

 scured by its association with the numerous other tracts of the 

 base of the mesencephalon. The caudal optic tract is 

 wholly distinct from the cephalic and, as in fishes, lies in a 

 groove beneath the tectum, whence it arches over the caudal as- 

 pect of the lobe. The lemniscus tracts, which seem always to 

 have been overlooked, have the usual course, springing from 

 the deeper layers of the tectum and accumulating, after a sep- 

 arate course caudo-ventrad, into a loose bundle which follow the 

 course of the caudal peduncle of the cerebellum. It would ap- 

 pear that Edinger's tractus ganglii habenulae ad mesencephalon 

 runs with or includes the cephalic brachium. Longitudinal 

 sections, however, clearly show that a fine-fibred tract arising 

 from the inner fibre layer of the tectum passes cephalad at some 

 distance from the median and enters the geniculatum. The re- 

 semblance between the optic tectum in Anura and turtles and 

 fishes may be seen by comparing Plate X, Fig. 10, Vol. Ill, 

 with Plate VII, Fig. i, and Plate X, Fig. 12, of Vol. II. This 

 structure is strikingly contrasted with that in Urodela. 



In Urodela the zone surrounding the common ventricle is 

 clothed with neurons whose processes extent peripherad and in- 

 terblend with the transversely or irregularly disposed neurons 

 which send axis cylinders into the optic tracts. The conditions 

 are much like those prevailing in very young fish [see Plate IX, 

 Figs. I, 2, 3. Vol. II.] 



The tracts to the leminscus blend with the optic nerve 

 tracts and brachia, while in the frog these several fibre layers 

 are differentiated as in reptiles and fishes. In higher reptilia 

 the relations are obscured by the complication of the tectum, as 

 is also the case in mammals. The fish, turtle or frog is there- 

 fore an incomparably better subject for study of the tectum 

 than the other groups, while the salamander is instructive as 

 showing how this complicated mechanism is derived from the 



