Johnston, Functional Divisions of Nervotis System. loi 



for comparison with structures in the cord or hind brain. Other 

 nuclei are said to receive the descending tracts from the corpus 

 striatum. These may probably be referred to the category of 

 commissural and tract cells. 



There remains to be considered the dorsal part of the brain 

 at its extreme rostral end. This region includes the dorsal half 

 of the primary fore brain. In its caudal part ('tween brain) 

 diverticula of the dorsal brain wall form one or more primitive 

 eyes, possibly paired. These are in all probability the most 

 primitive sense organs of the head region which still persist in 

 craniates. Since they are diverticula of the brain wall, neither 

 they nor their brain centers can be compared in any way with 

 either of the four chief divisions of the hind brain and cord. The 

 centers are either too far degenerated to be studied or are dom- 

 inated by other organs into whose service they have come. 



The chief part of the region under consideration is made 

 up of the olfactory apparatus. This is doubtless much younger 

 phylogenetically than the pineal structui"es, but also much more 

 important. The olfactory nerve has been compared in various 

 ways with the typical cranial and spinal nerves. The tendency 

 in late years has been to abandon this comparison, and an ex- 

 amination of the peripheral and central features of the olfactory 

 aparatus will show that the attempt to bring it into the same 

 category with the typical cranial nerves is wholly fruitless. The 

 cells from which the fibers of the olfactory nerve arise are situ- 

 ated in the epidermis, while the ganglion cells of the typical 

 nerves have been derived from the cord or brain. Evidence of 

 this is found in Amphioxus, where the sensory fibers are sent 

 out from cells lying in the cord, and in the giant ganglion cells 

 of the cord of fishes, which have the same relation (5, 6, 11). 

 These facts show that the olfactory nerve is the only one in 

 which fibers run from peripheral sense cells into the central sys- 

 tem. In all other cases, cells in the central system (or derived 

 from it) send fibers out to end in relation with sense cells in the 

 epidermis. The olfactory fibers on entering the brain break up 

 immediately in the glomeruli, in contrast to the conduct of the 

 typical sensory root fibers which run a longer or shorter dis- 



