98 Journal of Comparative Neurology. 



disappearance of dorsal roots would leave commissural and 

 tract cells without function. It seems possible that some of 

 these have acquired secondary functions which have prevented 

 their disappearance. 



The disposition of the secondary central tracts constitutes 

 a distinction between the somatic and splanchnic sensory 

 divisions which the writer has pointed out (12) and wishes to 

 emphasize here. The somatic sensory nuclei send internal 

 arcuate fibers to the tectum ; the splanchnic centers send 

 their fibers as an uncrossed secondary vagus tract to a nucleus 

 at the anterior end of the medulla. The connections of the 

 splanchnic central apparatus are imperfectly known, but it is 

 clear that they are radically different from those of the somatic 

 centers. It is an important part of the conception set forth in 

 this paper that the somatic and splanchnic sensory divisions are 

 distinct in their central relations. It is obvious that if both 

 sent their secondary tracts to a common center they could not 

 perform separate functions. The central reflex apparatus is not 

 less significant and important than the peripheral distribution of 

 the nerve components. 



The attempt to reduce the mid, 'tween, and fore brain to 

 terms of these four functional divisions leads us into a field 

 where few data for certain conclusions are at present to be found. 

 In the base of the mid brain, however, as noted above, the 

 somatic motor center is well developed and it extends forward 

 in the thalamus nearly to the rostral end of the brain axis. 

 Similarly the tract and commissural cells are present in the base 

 and lateral walls of the mid brain. Neither the sensory nor the 

 motor splanchnic column is represented rostral to the medulla. 

 If any further comparison is to be found between the mid brain 

 and the chief divisions of the cord and medulla, it must exist 

 between the tectum and the somatic sensory centers. Since 

 the retina is a part of the brain wall, the optic nerve is to be 

 regarded as a central tract and the tectum, so far as it is related 

 to the retina, as a coordinating or distributing center between 

 other brain centers. But the paired eyes are phylogenetically 

 younger than the brain centers with which they are connected, 



