192 



THE CEREBRAL HEMISPHERES. 



the anterior roots, and (2) the paired posterior spinal arteries, similarly derived from 

 the vertebrals, intercostals, and other arteries, and running just in front of the line 

 of attachment of the posterior roots. Another small longitudinal anastomotic chain 

 formed by branches of the posterior spinal runs along just behind the line of the 

 posterior roots. 1 The branches of these vessels ramify in the pia mater investing the 

 cord, communicating with one another to form transverse anastomoses, and from the 

 main vessels and their ramifications vessels pass in to supply both the grey and 

 white substance. 



The small entering branches may be described as forming two systems, a 

 centrifugal and a centripetal. The first is composed of a series of arterioles, 

 (central arterioles, Ross), 200 to 300 in number, which pass from the anterior spinal 



Fig. 135. SEMIDIAGRAMMATIC REPRESENTATION 



OF THE ARTERIES IN THE INTERIOR OF A 

 SEGMENT OF THE SPINAL CORD. (Ober- 



steiner. ) 



a.s.a., anterior spinal artery; c, a central 

 arteriole ; a, an anastomotic branch uniting it 

 with another arteriole of an adjacent segment ; 

 C'l., branch to Clarke's column ; a.p.f., artery 

 of posterior fissure ; p.m.a., posterior mesial 

 artery; a.p.e., artery of posterior cornu ; p.s.a., 

 branch of posterior spinal artery passing into 

 gelatinous substance ; p, other peripheral or 

 centripetal arterioles passing through white 

 substance of cord. 



artery into the anterior median fissure, 

 penetrating to the anterior commissure. 

 Here each one passes either to the right 

 or left, and divides into smaller arteries 

 and capillaries for the central parts of 

 the corresponding crescent of grey 

 matter; but a considerable ascending 

 and descending ramuscule is also given 

 off, and these overlap in their distri- 

 bution the corresponding longitudinal 

 branches of the adjacent central arte- 

 rioles. Although mainly distributed to the central parts of the grey matter the 

 central arterioles may also send branches to join the capillaries of the white matter. 

 The second or centripetal set has a converging or radial arrangement, passing in 

 from the periphery. Some of these simply form capillary loops, which supply the 

 superficial layers of the cord. Others are distributed to the white matter, where they 

 form comparatively large-meshed longitudinal plexuses. But the most considerable 

 of the centripetal arteries penetrate to the grey matter, and pour their blood into the 

 close capillary network which pervades it, supplying the parts not served by the 

 centrifugal vessels. The capillaries of the substantia gelatinosa are less numerous 

 than in the rest of the grey matter, and their meshes are mostly longitudinal. 



Special mention may be made of a series of small median arterial branches which 

 enter the posterior fissure, penetrating in it to the posterior commissure, and giving 

 off branches which supply the adjacent parts of the posterior white columns and 

 Clarke's column, where this is found ; and of the vessels which enter the grey matter 

 with the bundles of the anterior and posterior nerve-roots, and are distributed to the 

 corresponding cornua. It would appear however that no one part is supplied by only 

 one set of arterioles, nor is any one set of arterioles confined in its distribution to 



1 For the origin and course of the spinal arteries see Vol. II., pp. 421 and 422. 



