574 NERVOUS SYSTEM. 



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 main- 

 taining 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. 320) 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 mem- 

 brane. 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 near 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 ence- 

 phalon being about one to thirty-three. It does not nearly fill the canal in 

 which it is contained, its investing membranes being separated from the sur- 

 rounding 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, which is continued for some 

 distance into the filnm terminale. 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 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 some- 

 what in iloxion 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 lumbar. The cervical enlargement, which is the 

 larger, extends from the third cervical to the first dorsal vertebra: its greatest 

 diameter is in the transverse direction, and it corresponds with the origin of the 

 nerves which supply the upper extremities. The lower, or lumbar enlarge- 

 ment, is situated opposite the last dorsal vertebra, its greatest diameter being 

 from before backwards. It corresponds with the origin of the nerves which 

 supply the lower extremities. In form, the spinal cord is a flattened cylinder. 

 It presents, on its anterior surface, along the middle line, a longitudinal fissure, 



