372 



nearly to the middle line and are lost. The plane of sections is not 

 favorable for tracing them farther, but so far as they are followed 

 they correspond in position very closely to the radix mesencephalica 

 in Scyllium. 



Wallenberg seems not to have tried to obtain degeneration 

 of this tract by cutting the trigeminus roots. The relations in fishes 

 and amphibia are so clear that there is good reason for expecting 

 that the tract will prove to be sensory in all vertebrates. On the 

 other hand, the degeneration which Wallenberg did get indicates 

 that descending fibers exist in the tract. These fibers probably end 

 in the motor nucleus of the trigeminus and may perhaps be compared 

 with the short motor connections which are known to go from the 

 somatic sensory centers in the medulla oblongata and cerebellum to 

 the nuclei of the cranial motor nerves. These short motor tracts run 

 next the ventricle. After giving fibers to the motor nucleus the bundle 

 in birds goes into the motor root as in mammals. 



If it be admitted that this radix mesencephalica contains sensory 

 fibers, the fact is of great importance for our conception of the fun- 

 damental structure of the brain. The writer has shown reason (9, 

 10, 12) for regarding the tectum opticum of vertebrates as a primary 

 cutaneous sensory center which originally received the root of the pro- 

 fundus nerve. This root has shifted to a more caudal segment and 

 the tectum has become more or less modified. The evidence for this 

 interpretation of the tectum has been indirect, but now the simple 

 statement can be made that the root of the mesencephalon receives 

 sensory root fibers from the trigeminus and is therefore a primary 

 cutaneous sensory center. This is true in fishes and amphibia and 

 probably in all vertebrates, including man. 



The fact that the tectum opticum is even now a primary cutaneous 

 center very strongly supports the interpretation of the eye recently 

 suggested by the writer (12). On the basis of physiological, embryo- 

 logical and anatomical evidence which will not be repeated here, the 

 conclusion was reached that the retina represents a general cutaneous 

 ganglion together with the primary cutaneous center with which that 

 ganglion was originally related. So far as that argument rested on 

 the interpretation of the tectum as a general cutaneous center it is 

 inestimably strengthened by the new facts. The reason why a frog 

 orients itself in the same manner to light rays falling on the skin as 

 to those falling on the eye (17) is now more apparent. The skin and 

 the eye are related in part to identical central apparatus; the re- 

 lation is even closer than was formerly supposed. Further, the secon- 



