THE NERVES. 379 



transverse porous commissura alba, but upon that of the deeper 

 posterior fissure, gray substance (comm. grisea) which is situated in 

 the interior of the spinal marrow. This posterior fissure is, 

 according to Arnold and Valentin, in the fresh medulla, merely a 

 groove which only above and below passes into a fissure. We may 

 regard the spinal cord as consisting of two lateral halves. Each 

 half is divided by a sulcus lateralis (for the posterior roots of the 

 spinal nerves) into a larger anterior and a smaller posterior co- 

 lumn, between which and the anterior roots of the spinal nerves, 

 even a yet smaller, central, may be detected. In the commence- 

 ment of fetal life a canal is found in each half. 



Structure of the Spinal cord. White substance forms the ex- 

 terior, cortex, of the spinal cord, and gray the nucleus. 



The gray substance consists of ganglion globules (see before). 

 It is very vascular, of various colours (darker in children), and 

 presents, upon a transverse section, generally, this figure )(, above 

 and below rather ^-shaped ; hence in the gray substance a centre 

 and cornua, or crura, are described. The cornua are directed out- 

 wards towards the roots of the spinal nerves, lying, however, 

 closer to the posterior surface. 



The medullary substance consists of longitudinal fibres (nerve 

 tubules) which pass close to, but independently of, one another, are 

 separated by sheaths (pia mater), at least the larger, from each 

 other, forming broader or narrower fasciculi, and at last the two 

 (three) columns of either side, observable externally. According 

 to Valentin, the fibres are the immediate continuations of the spinal 

 nerves, which pass upwards, and the medullary substance contains 

 no more fibres than those taken together. Free extremities are 

 not found. Above (see Medull. Oblong. 618.) the posterior co- 

 lumns, divided into three to four fasciculi, go forwards, obliquely 

 through the gray substance, and proceed onwards as the anterior 

 pyramids, since the fasciculi of the two sides decussate (decussa- 

 tio), whilst the anterior columns pass directly into the posterior 

 pyramids (corpp. restiform.) 



628. The membranes of the spinal cord, the immediate con- 

 tinuations of the cerebral membranes. 



1. Dura mater, a long fibrous cylinder, extending fromforam. 

 magn. to the termination of canal, sacralis. The external surface 

 is separated from the vertebral canal by means of uniting and adi- 

 pose tissue, in which the plexus venosi spinales (interni) is found. 

 It does not perform the office of periosteum to the vertebrae. The 

 internal surface is covered with the arachnoid, and connected in 



