Johnston, Fimctional Divisions of Nervous System. 103 



missure. The secondary nuclei now send the stimuH to the 

 motor centers over three quite distinct paths. These paths are 

 shown in the accompanying figure. The first path consists of 

 direct tracts to ^:he ventral side of the rostral end of the brain, 

 the lobi inferiores, from which the impulses go over direct and 

 crossed tracts to the bulbar motor centers. The second and 

 shortest path is by way of the epistriatum and striatum 

 to the nucleus of the somatic motor fasciculus. It is pos- 

 sible that this path has another branch in the thalamus 

 by way of the central grey belonging either to the cate- 

 gory of commissural and tract cells or to the nucleus of the 

 posterior commissure. The third path is by way of the 

 ganglia habenulae and the bundles of Meynert. The first 

 step of this path is the tractus olfacto-habenularis which con- 

 nects dorsal grey masses of the fore brain with dorsally sit- 

 uated grey masses of the 'tween brain. However, the ganglia 

 habenulae are separated by the choroid plexus and it may be 

 that they are not morphologically dorsal, but lateral. The tracts 

 decussate in part or wholly at the dorsal surface of the 'tween 

 brain before ending (commissura superior). The second step 

 in this path connects the ganglia habenulae (after decussating) 

 with a special nucleus in the base of the mid brain which shows 

 a close resemblance to a collection of commissural cells. The 

 cells of this nucleus, finally, send their neurites to the motor 

 centers of the opposite side of the medulla. 



The olfactory apparatus thus consists of a primary sensory 

 center, the lobe ; of secondary dorsal centers in the fore brain; 

 and of tertiary centers in the 'tween brain, one ventral, one in 

 the central grey, and one lateral or dorsal. The nucleus of the 

 second path in the central grey is at least in part motor, the 

 ventral nucleus has been interpreted above as a special collec- 

 tion of commissural and tract cells, while the dorsal path re- 

 quires the bundles of Meynert to connect the 'tween brain 

 center with a nucleus of the grade of commissural cells. Thus 

 the central path is shortest, the ventral next, and the dorsal 

 longest, but the ventral is probably the oldest or most primitive 

 and the most general in its scope. The path by way of the 



