chap, xviii.] CEREBR UM A ND CEREBELL UM. 151 



continuation of the one lining the fourth ventricle, 

 and this again being a direct continuation of the 

 central grey nucleus of the cord. Like the central 

 canal of the cord, also, the ventricles are lined with a 

 layer of ciliated columnar, or short columnar epithelial 

 cells. 



197. The blood-vessels form a denser capillary 

 network in the grey than in the white matter ; in the 

 latter the network is pre-eminently of a longitudinal 

 arrangement, i.e., parallel to the long axis of the 

 bundles of the nerve-fibres. In the grey cortex of 

 the hemispheres of the cerebrum and cerebellum, many 

 of the capillary blood-vessels have an arrangement 

 vertical to the surface, but are connected with one 

 another by numerous transverse branches. 



The blood-vessels of the brain are situated in 

 spaces, perivascular lymph-spaces, traversed by fibres 

 passing between the adventitia of the vessels, and 

 the neuroglia forming the boundary of the space. 

 There are no real lymphatic vessels in the grey or 

 white substance. 



198. The white matter consists of medullated 

 nerve-fibres, which like those of the cord possess no 

 neurilemma or nuclei of nerve corpuscles, and no 

 constrictions of Ranvier. The nerve-fibres are of 

 very various sizes, according to the locality. Divisions 

 occur very often. When isolated they show the 

 varicosities mentioned in the cord. 



The grey matter consists, like that of -the cord 

 and medulla, besides the neuroglia, of a very fine net- 

 work of elementary nerve-fibrils (Rindfleisch, Gerlach), 

 into which pass, on the one hand, nerve-fibres, and, 

 on the other, the branched processes of ganglion cells. 



With regard to the structure of the ganglion cells 

 of the brain and medulla, what has been mentioned 

 of the ganglion cells of the cord holds good as to 

 them. Like the former, those of the medulla and 



