328 NERVOUS SYSTEM. 



enlargement is in the transverse direction or axis of the spinal 

 marrow, but not so much in its thickness, and terminates gra- 

 dually both above and below. The medulla spinalis afterwards 

 continues small, with very slight undulations or nodosities, until 

 within three or four inches of its lower extremity, when it again 

 enlarges. The enlargement here, though sufficiently obvious, 

 is not equal in actual magnitude to that in the neck, and is the 

 place from which all the lunabar nerves and the three superior 

 sacral proceed. It is then brought gradually to a point some- 

 what blunted, which most commonly does not descend below 

 the first lumbar vertebra. The point is, in some rare cases, 

 bifurcated, and by a transverse fissure converted into a tu- 

 bercle. 



The spinal marrow, besides terminating so much above the 

 lower end of the spinal canal, is much smaller in its diameter, 

 even with the addition of its membranes, than the canal. This 

 circumstance prevails, especially in the neck, and in the loins, 

 where much motion is experienced; and, consequently, a provi- 

 sion is thus made against any injury to it from pressure. * 



The Medulla Spinalis is marked off, longitudinally, into two 

 symmetrical parts, by one fissure in front and another behind, 

 both of which extend its whole length, and are placed exactly 

 in its middle. The contiguous edges or surfaces of each of these 

 fissures adhere so, that it requires a slight maceration or dissec- 

 tion to render them evident. The posterior fissure is decidedly 

 deeper, especially at its upper part, than the anterior: but the 

 latter, in return, is somewhat broader. The difference in depth, 

 however, is unimportant, as subjects are frequently met with in 

 which it is not appreciable. 



Moreover, on each side of the medulla spinalis there is a la- 

 teral fissure. It is not precisely in the middle, but somewhat 

 posterior, and pentrates inwards and forwards. In many in- 

 stances it is merely a simple superficial depression, much less 

 deep than either of the former. It does not run the whole 

 length of the medulla spinalis, but terminates somewhere in the 

 upper part of its thoracic portion by joining with its fellow af- 



* The Spinal Marrow at the third mouth of the embryo extends to the end of 

 the coccyx ; it thn suddenly contracts to the second lumbar vertebra. The coc* 

 cyx is contemporaneously reduced from seven to four pieces, Serres, 



