58 



THE SUBDIVISIONS OF THE BRAIN. 



form the basal ganglia that He anderneath the lobes of the hemispheres, and 

 which may be said to form the floor of the prosencoel. (Figs. 57, 58.) 



Minute Structure. — The minute structure of this region has been worked 



out in some detail in Limulus. 

 Two great masses of neurones that 

 probably belong to the third pro- 

 cephalic neuromere are found on 

 the median, neural surface of the 

 brain, underneath the posterior 

 lobes of the cerebral hemispheres 

 (Fig. 49, ch., H.c.) Their neurites 

 extend caudad and outward, and 

 then cephalad, forming a large 

 part of the posterior cerebral ped- 

 uncle. On reaching the base of 

 the hemispheres, they spread out 

 into great fan-shaped masses that 

 penetrate into the cortex of every 

 lobe and convolution, except the 

 median or gustatory one. (Fig. 48 

 and 49, G.c.) They run parallel 

 with similar fibers arising from the 

 two clusters of association cells, 

 H.a.s. lying above the gustatory 

 lobes. They terminate in minute, 

 spherical masses of neuropile that 

 form an indistinct, sub-cortical 

 layer in each lobe, and near which 

 the neurites of the cortical, granule 

 cells terminate. Some of the fibers 

 appear to terminate between the 

 cortical cells. (Fig. 50.) Numerous 

 branches from these neurites ramify 

 in the forebrain commissure, c.o., 

 and in the cheliceral lobes, ch.l. 



The cheliceral lobes (Fig. 49, 

 ch.l.) are large spherical masses of 

 neuropile lying on the anterior 

 lateral margin of the cheliceral 

 neuromeres. Their lateral surface is covered with small cells, whose neurites 

 together with many others, ramify in their interior. The most conspicuous ones 

 are those belonging to the two sets of cerebral association cells, ch, H.c. and H.as, 

 and the terminal dendrites of the main gustatory tract, G.tr'. 



Fig. 48.- 



-Median surface of a model of the forebrain of a 

 young Limulus about four inches long. 



