STRUCTURE OF THE LOBES OF THE CEREBRUM. 409 



by a far larger mass of the projection system than the parietal, 

 and even yet more than the occipital and temporal lobes. 



On the other hand, certain portions of the optic thalami 

 and the other centres of origin of the optic tract are connected 

 (and perhaps in the latter case thus only are connected) with 

 the occipital and temporal lobes of the hemispheres. The 

 latter relation may be traced in fig. 243, where the tubercle of 

 the optic thalamus (Th') f the external and internal corpora 

 geniculata (ge and gi), and the corpora quadrigemina, with 

 their projection systems, are exposed in a horizontal section. 

 We may here see how in the Monkey the radiating fibres 

 proceeding from the occipital lobes converge (Om) in the 

 ganglia of origin of the tractus opticus, to form a remarkably 

 thick medullary lamina (optic radiations of Gratiolet) covered 

 by a large number of arcuate fasciculi. This mass associates 

 itself with the posterior and at the same time external fasciculi 

 of the crusta of the crus cerebri. As it will be demon- 

 strated that the most external fasciculus of the crusta passes 

 through the decussation of the pyramids of the medullary 

 oblongata into the posterior column of the spinal cord, it is 

 obvious that the posterior roots of the spinal nerves are repre- 

 sented in the same lobes of the hemispheres as the radiations 

 of the optic tract. Gratiolet has at the same time shown that 

 the projection system, which according to his view proceeds 

 from the central cortex to the optic tract, but which really 

 penetrates its centres of origin (thalami), as well as the 

 radiating fibres issuing from the cortex and entering the outer- 

 most fasciculus of the crusta, by no means exclusively arise 

 from the occipital lobes, but also from the temporal lobes of 

 the hemispheres. With these conjoined fasciculi is associated 

 an extensive representation of still another sensory surface, 

 namely, of the olfactory mucous membrane. 



A considerable portion of the white substance of the olfac- 

 tory lobe (fig. 238, m 2 c) traverses the mass of the corpus 

 striatum, and extends as far as the transverse section of the 

 anterior commissure (c), which meets it in the opposite direc- 

 tion. This passage of the white substance of the olfactory 

 lobe into the anterior commissure has the form of an arch 

 convex outwards, which in the figure is for a short distance 



