DISSECTION OF THE BACK AND THORAX. 



141 



foramen iu 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 hgament. These receive also veins 

 from the vertebral bodies, and they are drained by vessels that issue by 

 the intervertebral foramina to join the vertebral, intercostal, lumbar, or 

 lateral sacral veins. 



Directions. — For the examination 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 hardening fluid for at least a week. 

 A portion from the spinal cord of any of the domestic animals will serve 

 the purpose ; but, from its larger size, that of the horse is to be pre- 

 ferred. 



Structure 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 three fissures, and a 

 fourth is sometimes de 

 scribed. The supeHofr med 

 ian fissure is a narro^\ 

 interval extending into the 

 cord along the middle line 

 of its upper face. It is 

 occupied by neuroglia. The 

 inferior median fissure \^ an 

 actual cleft penetrating the 

 cord along the middle line 

 of its lower face. It is 

 occupied by a process of 

 pia mater. The supero- 

 lateral fissure is a faint 

 surface depression extend- 

 ing on the side of the cord, 

 along the line of emergence 

 of the superior roots of the spinal nerves. 



Fig. 17. 



Teansverse Section of Spinal Cord of Calf {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 fissure ; 6. White (inferior) commissure ; 

 7. Inferior column of white matter ; 8. Inferior nerve roots ; 

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

 Superior column of white matter ; 12. Superior nerve roots. 



The infero-lateral fissure has 



