THE CEREBRUM. 



307 



cerebral cortex. Since, moreover, the cells of the latter send cortico- 

 thalamic fibres centrally, the intricate character of the connections of the 

 thalamus, which also gives off many additional fibres to lower lying levels, 

 is evident. 



Blood-Vessels of the Brain. The arteries supplying the brain 

 are derived from the internal carotid and vertebral stems. Their immediate 

 distribution to the cerebral and cerebellar cortex is everywhere through the 

 agency of the pia mater, within which the larger trunks, after frequent anas- 

 tomoses, give off the abundant small end-arteries that penetrate the subja- 

 cent nervous substance. On entering the gray matter, these cortical 

 branches, whose general course is parallel to one another and at right angles 

 to the surface, break up into rich networks of capillaries coming into direct 

 relation with the nervous elements. The vascular supply of the gray matter 

 is more generous than that of the white substance, which latter receives, 

 however, in addition to continuations from the cortical capillary network, a 

 number of medullary branches. These contribute few twigs to the gray 

 substance but traverse the latter and have their chief distribution as long- 

 meshed capillary networks within the . white matter. The arteries are 

 accompanied by connective tissue envelopes, prolonged from the pia mater, 

 which enclose ensheathing perivascutar lymph- spaces. 



The cortical veins begin in the white matter and pass through the 

 gray sheet to reach the pia mater, within whose external part they ramify, 

 the arteries usually lying deeper. The larger emergent stems, however, do 



Gray matter 



White matter 



F'G. 353. Injected dentate nucleus of cerebellum, showing rich capillary supply of plicated gray matter. 



X 20. 



not follow the main branches of the cerebral arteries, but converge towards 

 the lines of the principal adjacent dural sinuses into which they open. The 

 cerebral veins are among those possessing little or no muscular tissue and 

 no valves. 



True lymphatics are found neither within the brain nor spinal cord. 

 Lymph-paths, however, are represented by the peri-vascular sheaths sur- 

 rounding the blood-vessels within the nervous substance; these tracts com- 

 municate with the subarachnoid space. The pericellular spaces enclosing 

 the larger nerve-cells, as well as uncertain siibpial spaces between the pia 

 mater and the surface of the nervous substance, are closely related to the 

 lymphatic system, although not directly communicating with it. 



