792 



THE NERVOUS SYSTEM 



The axones descending from the cerebellum and the brain-stem are so mixed with other 

 axones that it is difficult to determine the sequence of their meduUation. The fasciculi contain- 

 ing them also contain axones of the variety in the fasciculi proprii and so show medullation early. 

 It is probable that the ascending, spino-cerebellar, fibres acquire their myeUn earUer than the 

 descending, if descending exist. 



Blood Supply of the Spinal Cord 



The spinal rami of the sacral, lumbar, intercostal, or vertebral arteries, as the case may be, 

 accompany the spinal nerves through the intervertebral foramina, traverse the dura mater 

 and arachnoid, and each divides into a dorsal and a ventral radicular artery. These accompany 

 the nerve-roots to the surface of the cord, and there break up into an anastomosing plexus in 

 the pia mater. From this plexus are derived three tortuously coursing longitudinal arteries 

 and; numerous independent central branches, which latter penetrate the cord direct. Of the 

 longitudinal arteries, the anterior spinal artery zigzags along the anterior median fissure and 

 gives off the anterior central branches, which pass into the fissure and penetrate the cord. These 

 branches give off a few twigs to the white substance in passing, but their most partial distribu- 

 tion is to the ventral portion of the grey substance. The two posterior spinal arteries, one on 

 each side, course near the lines of entrance of the dorsal root-fibres. They each branch and 

 anastomose, so that often two or more posterior arteries may appear in section upon either side 



Fig. 623. — Semi-diagrammatic Representation of the Blood Supply of the Spinal Cord , 



Posterior external spinal veins 



Posterior radicular vein /~~~~— -. Posterior central artery and vein 



Posterior spinal artery 



Anterior radicu- 

 lar artery 



Spinal ramus 



^ Internal spinal vein 

 ! I Anterior central artery 

 Anterior radicular vein Anterior external i Anterior spinal artery 

 spinal veins Anterior central vein 



of the dorsal root. These give off transverse or central twigs to the white substance, but espe- 

 cially to the grey substance of the dorsal horns. Of the remaining central branches many enter 

 the cord along the efferent fibres of the ventral roots, and are distributed chiefly to the grey 

 substance; others from the peripheral plexus throughout penetrate the cord and break up into 

 capillaries within the white substance. Some of the terminal twigs of these also enter the grey 

 substance. The blood supply of the grey substance is so much more abundant than that of 

 the white substance that in injected preparations the outline of the grey figure may be easily 

 distinguished by its abundance of capillaries alone. The central branches are of the terminal 

 variety. In the white substance the capillaries run for the most part longitudinally, or 

 parallel with the axones. 



The venous system is quite similar to the arterial. The blood of the central arteries is col- 

 lected into corresponding central venous branches which converge into a superficial venous 

 plexus in which are six main longitudinal channels, one along the posterior median sulcus, one 

 along the anterior median fissure, and one along each of the four lines of the nerve-roots. 

 These comprise the posterior and anterior external spinal veins (fig. 623). 



The internal spinal veins course along the ventral surface of the grey commissure, and arise 

 from the convergence of certain of the twigs of the anterior central vein. The posterior central 

 vein courses along the posterior median septum in company with the posterior central artery, 

 and empties into the median dorsal vein. The venous system communicates with the coarser 

 extra-dural or internal vertebral plexus chiefly by way of the radicular veins. 



II. THE BRAIN OR ENCEPHALON 



The brain is that greatly modified and enlarged portion of the central nervous 

 system which is enclosed within tiie cranial cavity. It is surrounded and sup- 

 ported by the same three membranes (meninges) that envelop the spinal cord. 



