160 THE NERVOUS SYSTEM. 



ately under the floor of the common ventricle, forming in their 

 course outwards a curve, with the concavity directed downwards. 

 This commissure connects the ' round bundles ' of opposite sides, 

 and those fibres coursing with the ' round bundles ' to the olfactory 

 lobes constitute the pars olfactoria of the commissura anterior. To 

 this commissure must also be added some fibres found on the ventral 

 surface of the commissura anterior and connecting the two strands 

 of the Tuber-cinereum an unusually coarse strand of these fibres 

 can be traced to the inner wall of the ventricle, and is termed the 

 pars olfactoria interna by Osborne. 



The general structure of the olfactory lobes resembles that of 

 the hemispheres ; the olfactory nerves arise each by two roots, an 

 outer and inner. The outer root arises from the outer wall near the 

 groove between the corresponding hemisphere and olfactory lobe ; 

 the inner or anterior root arises from the anterior surface of the 

 olfactory lobe. Both roots have a peculiar method of origin from 

 the extremely fine fibrillar network of the matrix (Nerve-fibre- 

 conglomerate, Koppen), in which are rounded dark bodies known 

 as ' glomeruli ; ' in the l glomeruli ' dark points and nuclei are seen, 

 between larger and smaller bands of nerve-fibres. Koppen holds 

 that all the sensory nerves of the brain arise in a similar manner. 



A decussation takes place between the two inner roots of the 

 olfactory nerves; possibly the external roots are connected by 

 means of the commissv.ra anterior. 



Very little pigment exists in the cerebral hemispheres or olfac- 

 tory lobes, the greater portion is found in the upper part of the 

 inner walls of the cerebral hemispheres. 



The epithelium of the ventricles of the brain, like that of the 

 central canal of the spinal cord, consists of conical cells with 

 their bases directed towards the cavity, and their apices directed 

 peripherally and prolonged into distinct processes (Figs. 104, 109 a). 

 In such situations, as the choroid plexuses, where nervous tissue is 

 absent and the cavity is completed by pia mater alone, the epithelial 

 cells are flattened. Everywhere else it is ciliated 1 and possesses dis- 

 .tinct round nuclei which are as broad as the cells themselves. The 

 epithelium is somewhat irregularly and sparsely pigmented ; the 

 ventral parts of the central canal of the spinal cord, of the fourth 

 ventricle, and of the Sylvian aqueduct are always more pigmented 

 than the dorsal parts. 



1 Schmidt (I. c.) states that the epithelium of the central canal of the spinal cord is 

 not ciliated. 



