144 



THE ANATOMY OF THE HORSE. 



the bifurcation of the cerebro-spinal artery. The middle spinal artery 

 passes backwards beneath the inferior median fissure of the cord. Its 

 branches are distributed to the cord and its membranes. As it passes 

 backwards giving off its branches it is reinforced by other arteries 

 entering at the intervertebral foramina. Thus, at each intervertebral 

 foramen in the neck a branch of the vertebral artery enters the spinal 

 canal. In the back similar branches enter from the dorso-spinal division 

 of the intercostal arteries, in the loins the branches emanate from the 

 lumbar arteries, and in the sacral region from the lateral sacral artery. 

 As a rule, the branches entering by adjacent foramina anastomose, and 

 then give off branches to the cord and the vertebral bodies. 



The Veins of the Spinal Cord are tortuous, and form on its surface 

 a plexus from which the blood passes into two large veins that lie one at 

 each side of the superior common ligament. These also receive veins 



from the vertebral bodies, 

 ^ ' and they are drained by 



vessels that issue by the 

 intervertebral foramina to 

 join the vertebral, inter- 

 costal, lumbar, or lateral 

 sacral veins. 



Directions. — For the ex- 

 amination of the structure 

 of the spinal cord, a few 

 inches of it with the roots 

 of the nerves intact should 

 be procured, and kept in 

 spirit or some other harden- 

 ing fluid for at least a week. 

 A portion from the spinal 

 cord of any of the do- 

 mestic animals will serve 

 the purpose; but, from 

 its larger size, that of 

 the horse is to be pre- 

 ferred. 



Stedcturb of the Spinal 

 Cord. The meninges having been removed, the student will note the 

 following points regarding the surface of the cord :— It approaches the 

 cylindrical in form, but is slightly flattened above and below. It is 

 traversed in the longitudinal direction by two median fissures, and 

 two lateral fissures are generally described as being present on each 

 half of the cord, though only one of them is distinguishable as a surface 

 depression. The superior median fissure is a narrow interval extending 



Fig. 19. 

 Tka.n'sveese Section or Spinal Cord of Cai.f (Klein). 



1. Superior median fissure ; 2. Central canal, in grey 

 (superior) commissure ; 3. Superior horn of grey matter ; 

 4. Inferior horn of grey matter ; 5. Process of pia mater 

 in inferior median Assure ; 0. White (inferior) commissure ; 

 7. Inferior column of white matter : S. Inferior nerve-roots ; 

 0. Lateral column of white matter ; 10. Pia mater ; 11. 

 Superior column of white matter ; 12. Superior nerve- 

 roots. 



