626 



STRUCTURE OF THE SPINAL CORD. 



The Spinal Cord. 



359l. STRUCTURE OF THE SPINAL CORD. [The key to the study of 

 the central nervous system is to remember that it begins as an involution of the 

 epiblast, and is originally tubular, with a central canal, dilated in the brain-end 

 into ventricles. In the spinal cord there are three concentric parts : first, the 

 columnar ciliated epithelium, outside this the central grey tube, and covering in all, 

 the outer white conducting fibres (Hi//).] 



Structure. The spinal cord consists of white matter externally and grey matter internally. 

 [It is invested by membranes the pia mater, composed of two layers and consisting of con- 

 nective-tissue with blood-vessels, being firmly adherent to the white matter and sending septa 

 into the substance of the cord. Both layers dip into the anterior median fissure, and only the 

 inner one into the posterior median groove. The arachnoid is a more delicate membrane and 

 non-vascular, while the dura mater is a tough membrane lining the vertebral canal, and 

 forming a theca or protective coat for the cord ( 381).] The grey mater has the form 

 of two crescents )-( placed back to back [or a capital H], in which we can distinguish an 

 anterior (a) and a posterior horn (/>), a middle part, and a grey commissure connecting the two 

 crescents. In the centre of this grey commissure is a canal central canal which runs from 

 the calamus scriptorius downwards ; it is lined throughout by a single layer of ciliated 

 cylindrical epithelium, [in the foetus, the cilia not being visible in the adult], and the canal 

 itself is the representative of the embryonal "medullary tube" (figs. 440, 446). [The part of 



Fig. 440. 

 Transverse section of the spinal cord ; in the centre is the butterfly form of the grey matter 

 surrounded by white matter, p, posterior, and a, anterior, horns of the grey matter ; PR, 

 posterior roots ; AR, anterior roots of a spinal nerve ; A, A, the white anterior ; L, L, the 

 lateral ; P, P, the posterior columns. 



the grey commissure in front of this canal is called the anterior, and the part behind, the 

 posterior grey commissure. In front of the grey commissure, and between it and the base ot 

 the anterior median fissure, are bundles of white nerve-fibres passing in a horizontal or oblique 

 direction from the anterior column of one side to the grey matter of the anterior cornu of the 

 opposite side (fig. 440). These decussating fibres constitute the white commissure. ] 



The white matter surrounds the grey, and is arranged in several columns [anterior, lateral, 

 and posterior by the passage of the nerve-roots to the cornua (figs. 440, 446)]. Along the 

 anterior surface of the cord there runs a well-marked fissure, which dips into the cord itself, 

 but does not reach the grey matter, as a mass of white matter the white commissure runs 

 from one side of the cord to the other. Between this fissure, known as the anterior median 

 nssure, and the line of exit of the anterior roots of the spinal nerves, lies the anterior column 



