Herrick, Tactile Centers of Prionotus. 311 



functional unit, being more closely related with each other than 

 with any structures above or below them. Within the third lobe 

 the fasciculus dorso-lateralis rapidly increases again in size and a 

 lateral protrusion of this lobe receives the dorsal root of the second 

 spinal nerve (Fig. 6). 



Between the second and third lobes the dorsal root of the 

 second spinal nerve enters and distributes to both of these lobes. 

 Between them the dorso-lateral fasciculus also increases to its 

 maximum size, composed largely of dorsal root fibers of the 

 second spinal nerve, some of whose fibers extend forward to the 

 first lobe. 



The first and second lobes are somewhat smaller than the others 

 and are but indistinctly separated by a wide shallow groove. At a 

 point near the cephalic end of the second lobe the grey mass of the 

 lobe (dorsal cornu), which behind this point is separated like the 

 other lobes from the lobe of the opposite side by a deep dorsal 

 fissure, fuses across the median line (Fig. 7). This fusion, which 

 at first involves only the second lobe, a little farther forward 

 becomes overlapped by a similar fusion of the first lobe, the second 

 lobe being thrust far forward embedded under the first (Fig. 8). 

 In this way there are formed two commissural grey masses, a 

 dorsal connected with the first lobe, and a ventral connected with 

 the second lobe, both lying dorsally to the ventricle and both 

 containing cells and medullated and unmedullated commissural 

 fibers. These median grey masses constitute the somatic commis- 

 sural nucleus and the fibers which decussate associated with them 

 the somatic element of the commissura infima Halleri. The com- 

 missure is continuous caudad with the dorsal inter-lobar commis- 

 sure already described. It is considerably enlarged at its cephalic 

 end just behind the membranous roof of the fourth ventricle (Fig. 

 II). The somatic commissural nuclei in Weigert preparations 

 resemble closely in structure the accessory lobes with which they 

 are directly connected. 



The first accessory lobe functions also as the nucleus of the 

 spinal V tract. It is related with the fasciculus dorso-lateralis 

 and with the scattered tracts which I have designated funiculus 

 dorsalis and so may be regarded as sharing some of the functions 

 of the nucleus funiculi, though the latter structure is separately 

 represented in a well defined nucleus associated with the fascic- 

 ulus dorso-lateralis. 



