136 ANATOMY OP UPPER EXTREMITY, ETC. 



the spinal canal by anastomoses from the vertebral arteries 

 in the neck, and from the intercostal and lumbar arteries. 

 The posterior spinal artery is also derived from the same 

 source, and is continued down the posterior aspect of the 

 spinal canal by anastomoses from the same branches that 

 reinforce the anterior spinal artery. 



The veins of the spinal cord are, very tortuous, and form 

 a plexus on its surface, emptying their contents into the 

 vertebral, intercostal, lumbar, and sacral veins. 



The spinal cord should be hardened in alcohol for examination, as, 

 soon after death, it becomes softened, and unfit for dissection. 



The SPINAL CORD extends from the medulla oblongata to 

 the first or second lumbar vertebra. In shape it is a flat- 

 tened cylinder, and it has two enlargements, the superior 

 corresponding to the origin of the nerves for the upper 

 extremity, and the inferior enlargement to that of those for 

 the lower extremity. The cord has a fissure along its ante- 

 rior surface, and another along its posterior surface ; the 

 anterior, called the fissura longitudinalis anterior, is the 

 widest, and the posterior, called the fissura longitudinalis 

 posterior, is the deepest ; a lateral fissure also exists along 

 the line of origin of the posterior roots of the spinal nerves, 

 and another has been described as being found along the 

 line of origin of the anterior roots. 



A transverse section of the cord will show that each of 

 its lateral halves is divided by the lateral fissure into two 

 parts, that in front of the fissure being called the antero- 

 lateral column, and that behind, the posterior column ; it 

 will also show that the two halves of the cord are united 

 hy a central portion which limits the depth of the longitu- 

 dinal fissures, and is called the commissure. 



A transverse section of the cord shows, also, that, like 

 the brain, it is composed of white and gray substance, but 

 the gray portion is surrounded by the white, instead of 

 being external, as in the encephalon. The gray matter is 

 arranged, in each half of the cord, in the form of a crescent, 

 the horns of which point toward the roots of the nerves ; 

 the convexity looks toward the commissure, which also is 

 chiefly made up of gray matter. The posterior horn of the 

 crescent reaches to the fissure along the attachment of the 

 posterior roots ; the anterior horn does not reach to the ante- 

 rior roots, nor does it form so sharp a point as the posterior. 



The deep origin of the spinal nerves, like that of the 

 cranial nerves, is uncertain. 



