710 



THE NERVOUS SYSTEM. 



operation is performed by inserting a trocar, of the smallest size, 

 between the lamina of the third and fourth or the fourth and fifth 

 liuiiliar vi.-rt<'ltr;u through the ligamenta subflava. The spinal 

 cord even of a child at birth does not reach below the third 

 lumbar vertebra, and therefore the canal may be punctured be- 

 tween the third and fourth vertebra without any risk of injuring 

 its contents. The point of puncture is indicated by laying the 

 child on its side and dropping a perpendicular line from the 

 highest point of the crest of the ilium ; this will cross the upper 

 border of the spine of the fourth lumbar vertebra, and will indi- 

 cate the level at which the trocar should be inserted a little to 

 one side of the median line. 



THE SPINAL CORD (Fig. 379). 



The Spinal Cord (medulla spinalis] is the cylindrical, 

 elongated part of the cereb'ro-spinal axis which is con- 

 tained in the vertebral canal. Its length is usually 

 about seventeen or eighteen inches, and its weight, 

 when divested of its membranes and nerves, about one 

 ounce and a half, its proportion to the encephalon being 

 about 1 to 33. It does riot nearly fill the canal in 

 which it is contained, its investing membranes being 

 separated from the surrounding walls by areolar tissue 

 and a plexus of veins. It occupies, in the adult, the 

 upper two-thirds of the vertebral canal, extending 

 from the upper border of the atlas to the lower border 

 of the body of the first lumbar vertebra, where it ter- 

 minates in a slender filament of gray substance, which 



Posterior 

 Median Fissure. 



Posterior 

 Lateral Fissure. 



FIG. 379. Posterior view of the 

 spinal cord in situ. 



FIG. 380. Spinal cord. Side 

 view. Plan of the .fissures and 

 columns. 



is continued for so.me distance into the filum terminate. 

 In the foetus, before the third month, it extends to the 

 bottom of the sacral canal, but after this period it grad- 

 ually recedes from below, as the growth of the bones 

 composing the canal is more rapid in proportion than 

 that of the cord, so that in the child at birth the cord 

 extends as far as the third lumbar vertebra. Its position 

 varies also according to the degree of curvature of the 

 spinal column, being raised somewhat in flexion of the 

 spine. On examining its surface it presents a difference 

 in its diameter in different parts, being marked by two 

 enlargements, an upper or cervical, and a lower or lum- 

 bar. The cervical enlargement extends from about the 

 third cervical to the first or second dorsal vertebra : its 

 greatest diameter is in the transverse direction (13 

 mm.), and it corresponds with the origin of the nerves 



