502 NERVOUS SYSTEM. 



sheath as far as their point of exit from the dura mater, where it becomes con- 

 tinuous with the parietal layer. 



The arachnoid is destitute of vessels. No nerves have as yet been traced into 

 this membrane. 



The Pia Mater of the cord is exposed on the removal of the arachnoid (fig. 253). 

 It is less vascular in structure than the pia mater of the brain, with which it is 

 continuous, being thicker, more dense in structure, and composed of fibrous tissue, 

 arranged in longitudinal bundles. It covers the entire surface of the cord, to 

 which it is very intimately adherent, forming its neurilemma, and sends a process 

 downwards into its anterior fissure, and another, extremely delicate, into the 

 posterior fissure. It also forms a sheath for each of the filaments of the spinal 

 nerves, and invests the nerves themselves. A longitudinal fibrous band extends 

 along the middle line on its anterior surface, called by Haller, the linea splendens ; 

 and a somewhat similar band, the ligamentum denticulatum, is situated on each 

 side. At the point where the cord terminates, the pia mater becomes contracted, 

 and is continued down as a long, slender filament (filum terminale}, which 

 descends through the centre of the mass of nerves forming the cauda equina, and 

 is blended with the impervious sheath of dura mater, on a level with the top of 

 the sacral canal. It assists in maintaining the cord in its position during the 

 movements of the trunk, and is, from this circumstance, called the central 

 ligament of the spinal cord. It contains a little nervous substance, which may 

 be traced for some distance into its upper part, and is accompanied by a small 

 artery and vein. 



Structure. The pia mater of the cord, though less vascular than that which 

 invests the brain, contains a network of delicate vessels in its substance. It is 

 also supplied with nerves, which are derived from the sympathetic, and from the 

 posterior roots of the spinal nerves. At the upper part of the cord, the pia 

 mater presents a grayish, mottled tint, which is owing to yellowish or brown 

 pigment cells being scattered within its tissue. 



The Ligamentum, Denticulatum (fig. 253) is a narrow, fibrous band, situated on 

 each side of the spinal cord, throughout its entire length, and separating the 

 anterior from the posterior roots of the spinal nerves, having received its name 

 from the serrated appearance which it presents. Its inner border is continuous 

 with the pia mater, at the side of the cord. Its outer'border presents a series of 

 triangular, dentated serrations, the points of which are fixed, at intervals, to the 

 dura mater, serving to unite together the two layers of the arachnoid membrane. 

 These serrations are about twenty in number, on each side, the first being attached 

 to the dura mater, opposite the margin of the foramen magnum, between the 

 vertebral artery and the hypoglossal nerve; and the last corresponds to nearly the 

 lower end of the cord. Its use is to support the cord in the fluid by which it is 

 surrounded. 



THE SPINAL CORD. 



The spinal cord (medulla spinalis) is the cylindrical elongated part of the cerebro- 

 spinal axis, which is contained in the spinal canal. Its length is usually about 

 sixteen or seventeen 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 not 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 spinal 

 canal, extending from the foramen magnum to the lower border of the body of 

 the first lumbar vertebra, where it terminates in a slender filament of gray 

 substance, the filum terminale, which lies concealed among the leash of nerves 

 forming the cauda equina. In the foetus, before the third month, it extends to the 

 bottom of the sacral canal; but, after this period, it gradually recedes from 

 below, as the growth of the bones composing the canal is more rapid in proportion. 



