Johnston, Morphology of the Head. 253 



but are neomorphs in craniates which are just beginning their 

 specialization in cyclostomes. 



The general cutaneous nerve of Locy and the olfactory 

 nerve are to be taken together as roughly the equivalent of the 

 nerve of the prostomium in segmented worms. 



77. The dorsal commissures of the bi'ain. 



In Figs. 1—7 are shown the longitudinal zones of the brain 

 belonging to the four functional divisions of the nervous system. 

 A further review of these zones does not fall within the scope 

 of the present paper. Some remarks upon the dorsal commis- 

 sures and their relations to the zones, however, will perhaps be 

 of value. It should be said at the outset that the fiber crossings 

 in the brain of lower vertebrates, both dorsal and ventral, are in 

 the greatest part if not wholly mere decussations. True com- 

 missures are almost unknown. When the word "commissure" 

 is used in the following paragraphs it is used because it has come 

 to be part of the accepted name of a given fiber-crossing. 



The dorsal decussation of the spinal cord in higher verte- 

 brates contain splanchnic sensory (sympathetic) fibers, collaterals 

 from cutaneous fibers, and perhaps secondary elements. At 

 the junction of the spinal cord and brain in all vertebrates this 

 decussation is greatly enlarged just behind the choroid plexus 

 of the IV ventricle. This enlarged portion, known as the com- 

 missura infima Halleri, is composed chiefly of splanchnic sen- 

 sory fibers from the roots of the VII, IX and X nerves. These 

 fibers for the most part end in the median nucleus of the com- 

 missure of Cajal (46, 6'], 68). A part of them without cross- 

 ing (and a part of the crossed fibers also?) pass on back into 

 the spinal cord (68). Other fibers in the commissura infima 

 come from the cells of the nucleus funiculi (90, 46). From 

 these facts it is evident that both splanchnic sensory and somatic 

 sensory elements cross in the dorsal decussation of the cord. 

 The somatic sensory elements are chiefly or wholly secondary 

 fibers, or only collaterals. 



These two components must be rigidly distinguished if we 

 are to understand the dorsal decussations of the brain. The 



