MORPHOLOGY OF SPINAL CORD 111 



In the study of the central nervous system its enveloping membranes or 

 meninges are met with first, and logically should be considered first, but since a 

 comprehensive description of these membranes involves a foreknowledge of the 

 various structures with which they are related, it is more expedient to consider 

 them after making a closer study of the entire system they envelop. 



For convenience of study, the central nervous system is separated into the 

 gross divisions, spinal cord and brain (encephalon) as illustrated in fig. 602. 

 Each of these divisions will be subdivided and considered with especial reference 

 to its anatomical and functional relations to the other divisions and the inter- 

 relations of its component parts. 



I. THE SPINAL CORD 



The spinal cord [medulla spinalis] is the lower (caudal) and most attenuated 

 portion of the central nervous system. It is approximately cylindrical in form 

 and terminates conically. Its average length in the adult is 45 cm. (18 in.) in the 

 male and 42 cm. in the female. It weighs from 26 to 28 grams or about 2 per 

 cent, of the entire cerebro-spinal axis. 



After birth it grows more rapidly and for a longer period than the encephalon, increasing 

 in weight more than sevenfold, while the brain increases less than haK that amount. Its specific 

 gravity is given as 1.038. 



The Une of division between the spinal cord and the medulla oblongata is arbitrary. The 

 outer border of the foramen magnum is commonly given, or, better, a transverse line just below 

 the decussation of the pyramids. Lying in the vertebral canal, the adult cord usually extends 

 to the upper border of the body of the second lumbar vertebra. However, cases may be found 

 among taller individuals in which it extends^no farther than the last thoracic vertebra. With 

 increase in stature, its actual length increases, but the extent to which it may descend the verte- 

 bral canal decreases. Up to the third month of intra-uterine life it occupies the entire length 

 of the vertebral canal, but owing to the fact that the vertebral column lengthens more rapidly 

 and for a longer period than does the spinal cord, the latter, being attached to the brain above, 

 soon ceases to occupy the entire canal. At birth its average extent is to the body of the third 

 lumbar vertebra. 



External Morphology of the Spinal Cord 



In position in the body, the spinal cord conforms to the curvatures of the 

 canal in which it lies. In addition to the bony wall of the vertebral canal, it is 

 enveloped and protected by its three membranes or meninges, which are con- 

 tinuous with the like membranes of the encephalon: first, the pia mater, which 

 closely invests the cord and sends ingrowths into its substance, contributing to its 

 support; second, the arachnoid, a loosely constructed, thin membrane, separated 

 from the pia mater bj- a considerable subarachnoid space; third, the dura mater, 

 the outermost and thickest of the membranes, separated from the arachnoid by 

 merely a slit-like space, the subdural space. 



The intimate association of the central system with all the peripheral organs is 

 attained chiefly through the spinal cord, and this is accomplished by means of 

 thirty-one pairs of spinal nerves, which are attached along its lateral aspects. The 

 nerves of each pair are attached opposite each other at more or less equal intervals 

 along its entire length, and in passing to the periphery they penetrate the men- 

 inges, which contribute to and are continuous with the connective-tissue sheaths 

 investing them. Each nerve is attached by two roots, an afferent or dorsal root, 

 which enters the cord along its postero-lateral sulcus, and an efferent or ventral 

 root, which makes its exit along the ventro-lateral aspect. 



With its inequahties in thickness and its conical termination the spinal cord is 

 subdivided into four parts or regions:— (1) The cervical portion, with eight pairs 

 of cervical nerves; (2) the thoracic portion, with twelve pairs of thoracic nerves; 

 (3) the lumbar portion, with five pairs of lumbar nerves; and (4) the conus 

 meduUaris, or sacral portion, with five pairs of sacral and one pair of coccygeal 

 nerves. From the termination of the conus meduUaris, the pia mater continues 

 below in the subarachnoid space into the portion of the vertebral canal not 

 occupied by the spinal cord, and forms the non-nervous, slender, thread-like 

 terminus, the jilum terminale. This becomes continuous with the dura mater at 

 its lower extremity. 



