762 TEE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



The neuroglia and nerve-tuhes constitute the ivhite substance, which Is 

 arranged, as we know, into three columns at least. All the tubes of this sub- 

 stance do not ascend to the brain, as was believed for a long time ; the opinion 

 that the tubes of the spinal nerves formed the medulla and extended to the brain, 

 has been abandoned since Volkmann measured, comparatively, the section of all 

 these nerves and that of the nervous spinal-axis. 



The tubes or fibres of the white substance have not always the same com- 

 position, nor the same size. Many have no enveloping membrane, and those 

 which possess one have not a nucleus ; so that it is difficult to regard this as a 

 true sheath of Schwann. Certain anatomists, however, consider them to be so, 

 but without the constriction (Ranvier) ; while others admit that they are so 

 provided, like the elements of the nerves (Tourneur and Legoff ). 



These tubes are thick in the inferior columns, and uniformly smaller in the 

 superior ones. A mixture of fine and thick tubes is observed in the lateral 

 columns, but the finest are always generally towards the grey substance.^ 



(Volkmann has established the fact, that the size of the medulla corresponds 

 with the number of nerve-tubes given off at any point. He gives the weight of 

 four segments, each 2f inches in length, from the spinal cord of the Horse, and 

 the relative extent of the grey matter in square lines ; these are as follows : — 



In the white substance the tubes are longitudinal, oblique, or transversal ; 

 the latter arise from the cells of the grey substance, and form the roots of the 

 nerves emerging either by the superior or inferior collateral fissure. 



The tubes of the inferior columns pass to the cells of the grey substance, or 

 reach the brain by remaining in the corresponding moiety of the cord — the 

 fibres of the right half of the medulla gaining the brain without passing into 

 the left half. Those of the lateral columns decussate, each sending to, and 

 receiving from, the other, tubes which cross in the white commissure. The 

 superior columns contain fibres that extend directly to the brain ; these are 

 sensorial. There are also found transverse fibres that enter the cells of the 

 superior grey cornua, and others that pass into the cells of the inferior or motor 

 cornua. 



Such is, in a few words, the arrangement of the nerve-elements in the spinal 

 cord. The subject is a very long and complicated one, which cannot be dealt 

 with in a more detailed manner in an elementary work on descriptive anatomy. 



Vessels. — In the spinal cord the grey is richer in vessels than the white sub- 

 stance. The capillary networks are finer in the former than the latter, and chiefly 

 in the vicinity of the nerve-cells. 



Arterial blood entere the cord by three orders of vessels : 1. The median 

 arteries, which pass into the inferior and superior middle fissures. 2. The radi- 

 cular arteries, which follow the roots of the spinal nerves. 3. The peripheral 

 arteries, which arise from the vascular ramifications of the pia mater (Duret). 

 The vessel which furnishes these divisions is the middle spinal artery. 



' There is a tendency now to the belief that these slender tubes serve for the conveyance 

 of sensory impressions. An alteration in tliem is coincident with deranged sensibility. 



