WHITE SUBSTANCE OF SPINAL CORD 777 



in the same level of the opposite side; (3) a lateral group of cells, sometimes separated into a dorso- 

 lateral and a ventro-lateral portion, occupies the lateral column or horn, and is best differentiated 

 in the cervical and lumbar enlargements. Most of the axones arising from its larger ceUs are 

 contributed to the ventral root of the same side, and such axones probably supply the muscles 

 of the extremities. Some of those from its ventral portion are distributed to the muscles of 

 the body-wall; the dorso-lateral portion is that part of the lateral column which persists 

 throughout the cord, and is considered as supplying the visceral efferent fibres in the ventral 

 roots. (4) an intermediate group, occupying the mid-dorsal portion of the ventral horn. 

 Axones arising from its cells are probably seldom contributed to the ventral root, but instead 

 course wholly within the central nervous system. Some pass to the opposite side of the cord, 

 chiefly via the anterior and possibly the posterior white commissure, to terminate either in the 

 same or different levels of the grey column. Others of longer course pass to the periphery of 

 the cord, join one of the spino-cerebellar fasciculi, and pass upward to the cerebellum. 



Furthermore, there are scattered throughout the grey substance many smaller cell-bodies 

 of neurones. These give rise to axones of shorter course, either commissural or associational 

 proper. Of such axones many are quite short, coursing practically in the same level as that in 

 which their cells of origin are located, and serve to associate the different parts of the grey sub- 

 stance of that level. Others course varying distances upward and downward for the association 

 of different levels of the grey column. 



It is evident from the above that in addition to the various nerve-cells it contains, there is 

 also to be found a felt-work of axones in the grey substance. Many of these axones are 

 meduUated, though not in sufficient abundance to destroy the grey character of the substance. 

 The felt-work is composed of three general varieties of fibres: — (1) The terminal branches of 

 axones entering from the fasciculi of the white substance and forming end-brushes about the 

 various ceU-bodies in the grey substance (partly medullated); (2) axones given off from the cells 

 of the grey substance and which pass into the surrounding white substance either to enter the 

 ventral-roots or to join the ascending and descending fasciculi within the spinal cord (partly 

 medullated); (3) axones of Golgi neurones of type II, which do not pass outside the confines 

 of the grey substance (non-meduUated). Some axones of any of these varieties may 

 cross the mid-line and thus become commissural. In general aU fibres of long course acquire 

 medullary sheaths a short distance from their cells of origin, and lose them again just before 

 termination. 



The white substance of the spinal cord. — The great mass of the axones of the 

 spinal cord course longitudinally and form the thick mantle surrounding the 

 column of grey substance. This mantle is divided into right and left homo- 

 lateral halves by the anterior median fissure along its ventral aspect, and along 

 its dorsal aspect by the posterior median septum, which is for the most part a 

 connective-tissue partition derived from the pia mater along the line of the 

 posterior median sulcus. The mantle is supported internally by interwoven 

 neuroglia and white fibrous connective tissue, the latter, derived chiefly from the 

 pia mater, closely investing it without. 



The axones of the white substance belong to three general neurone systems : — 

 (1) The spino-cerehral and cerebrospinal system, which consists of axones of long 

 course, one set ascending and another descending, forming links in the neurone 

 chains between the cerebrum and the peripheral organs. The ascending axones 

 of this system collect the general bodily sensations which are conveyed 

 to the cerebrum, the cells of which in response contribute axones which descend 

 the cord, conveying efferent or motor impulses. (2) The spmo-cerebellar and 

 cerebello-spiyial system consists of conduction paths, one set ascending and another 

 descending, which are connections between cerebellar structures and the grey 

 substance of the spinal cord. (3) The spinal association and commissural system 

 of axones which serve to assocm^e the different levels and the two sides of the spinal 

 cord and which are proper to the spinal cord, i. e., they do not pass outside its 

 confines. 



Both the first and second systems increase in bulk as the cord is ascended. 

 The ascending axones of each system are contributed to the white substance of the 

 cord along its length, and therefore accumulate upward; the axones descending 

 from the encephalon are distributed to the different levels of the cord along its 

 length, and therefore diminish downward. 



The mass of the third system of axones varies according to locality. Wherever 

 there is a greater mass of neurones to be associated, as there is in the enlargements 

 of the cord, a greater number of these axones is required. Their cells of origin, 

 being in the grey substance of the cord, contribute to its bulk and thus both the 

 cells and the axones of this system serve to make the enlargements more marked. 

 In the lumbar and sacral regions the greater mass of the entire white substance 

 consists of axones belonging to this system. It forms a dense felt-work about 

 the grey column throughout the cord. Necessarily this system contains axones 

 of various lengths. Some merely associate different levels within a single segment 



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