THE SPINAL CORD. 695 



Structure. The arachnoid is a delicate membrane made up of closely arranged 

 interlacing bundles of connective tissue in several layers. 



The Pia Mater of the cord is exposed on the removal of the arachnoid (Fig. 400). 

 It covers the entire surface of the cord, to which it is very intimately adherent, 

 forming its neurilemma, and sending a process downward into its anterior 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 xplendens ; and a some- 

 what similar band, the ligamentum dentu'idatum, is situated on each side. At 

 the point where the cord terminates the pia mater becomes contracted, and is con- 

 tinued 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 third sacral vertebra. 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 gray nervous substance, which may be traced for some dis- 

 tance into its upper part, and is accompanied by a small artery and vein. 

 At the upper part of the cord the pia mater presents a grayish, mottled tint, 

 which is owing to yellow or brown pigment-cells scattered among the elastic 

 fibres. 



Structure. The pia mater of the cord is less vascular in structure, but thicker 

 and denser, than the pia mater of the brain, with which it is continuous. It 

 consists of two layers : an outer composed of bundles of connective-tissue fibres, 

 arranged for the most part longitudinally ; and an inner, consisting of stiff 

 bundles of the same tissue, which present peculiar angular bends, and is covered 

 on both surfaces by a layer of endothelium. Between the two layers are a 

 number of cleftlike lymphatic spaces which communicate with the subarachnoid 

 cavity, and a number of blood-vessels which are enclosed in a peri vascular 

 sheath, derived from the inner layer of the pia mater, into which the lymphatic 

 spaces open. It is also supplied with nerves, which are derived from the sympa- 

 thetic. 



The Ligamentum Denticulatum (Fig. 400) 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. It has 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. These serrations are twenty-one 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. 4 



THE SPINAL CORD (Fig. 402). 



The Spinal Cord (medulla spinalis) is the cylindrical, elongated part of the 

 cerebro-spinal axis which is contained 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 encepha- 

 lon being about 1 to 33. It does not nearly fill the canal in which it is con- 

 tained, its investing membranes being separated from the surrounding walls by 

 art-' )lar 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 terminates in a 

 slender filament of gray substance, which is continued for some distance into the 

 Ilium 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 



