I030 HUMAN ANATOMY. 



of the last sacral nerve, it reaches three-fourths. The absolute amount of gray matter is greatest 

 within the cervical and lumbar enlargements of the cord, where it is directly related to the large 

 nerves supplying the limbs. On comparing the tracts of white matter and the gray column it 

 follows that while in the lower third of the lumbar cord these are of appro.ximately equal area, 

 below this level the gray matter exceeds the white. In the remaining regions, on the other 

 hand, the white matter predominates, in the greater part of the thoracic cord exceeding the gray 

 from four to five fold and in the cervical cord being from two to three times greater. 



The Central Canal. — Where well represented, the central canal (canalis cen- 

 tralis), the remains of the once conspicuous neural tube, appears as a minute 

 opening in the gray commissure, about . 2 mm. in diameter and barely visible with 

 the unaided eye. In the child it extends the entire length of the cord and, below, 

 ends blindly in the upper half of the filum terminale. Above, it opens into the lower 

 end of the fourth ventricle, from which it is prolonged downward through the lower 

 half of the medulla oblongata into the spinal cord. In not over one-fifth of adult 

 subjects, however, is the canal retained as a pervious tube throughout the cord, its 

 lumen usually being partially or completely obliterated for longer or shorter stretches, 

 the lumen last disappearing in the lower part of the cord. Within the conus 

 meduUaris, the central canal regularly exhibits an expansion, the sinus terminalis, 

 that begins below the origin of the coccygeal nerve and extends caudally for from 

 8-10 mm., with a maximum, frontal diameter of i mm. or over. 



The obliteration of the central canal, complete in about 50 per cent, of subjects beyond 

 middle life (Schulz), is to be regarded as a physiological accompaniment of advancing age. It 

 is effected by displacement and proliferation of the ependyma-cells lining the canal, in conjunc- 

 tion with ingrowth of the surrounding neurogliar fibres (Weigert). The form of the canal, as 

 seen in cross-sections, is very variable and uncertain owing tb the changes incident to the use 

 of hardening fluids. In a general way when well preserved the lumen is round or oval and 

 smallest in the thoracic region ; in some places, as in the upper cervical cord and in the lumbar 

 enlargement, it is larger and often appears pentagonal in outline, whilst in others the calibre 

 may be reduced to a sagittal slit. The position of the central canal varies at different levels in 

 relation to the ventral and dorsal surfaces of the cord. In the middle of the lumbar region it 

 occupies approximately the centre of the cord, but above, in the thoracic and cervical segments, 

 it lies much nearer the ventral than the dorsal surface, while below it gradually approaches the 

 dorsal surface, but always remains closed. 



Mention may be made of a remarkable structure named Reissner's fibre, after its discov- 

 erer, that as a longitudinal thread of great delicacy lies free within the central canal of the cord 

 and the lower ventricle of the brain, extending from the cavity of the mesencephalon above to 

 the lowest part of the cord-canal below. The interpretation of this structure as an artefact, 

 which considering its e.xtraordinary position is most natural, seems untenable in view of the 

 positive testimony, confirming its existence as a preformed and true structure in many 

 vertebrates, given by several subsequent observers and especially by Sargent. According to 

 Dendy and to Nicholls,' the fibre is concerned in automatically regulating flexion of the body, 

 by transmitting to the brain stimuli due to changes in tension. 



MICROSCOPICAL STRUCTURE OF THE SPINAL CORD. 



The three chief components of the spinal cord — the nerve-cells, the nerve-fibres 

 and the neuroglia — vary in proportion and disposition in the white and gray matter. 

 It is, therefore, desirable to consider the general structure of the cord before describ- 

 ing its detailed characteristics at different levels. 



The Gray Matter. — The most distinctive elements of the gray matter are the 

 multipolar nerve-cells which lie embedded within a complex sponge-like matrix formed 

 by the various processes— dendrites, axones and collaterals — from other neurones, the 

 supporting neuroglia and the blood-vessels. In two localities — immediately around the 

 central canal and capping the dorsal cornu — the gray matter varies in its appearance 

 and constitution and exhibits the modifications peculiar to the central and Rolandic 

 substantia gelatinosa, the details of which call for later description (page 1034). 



The nerve-cells of the anterior horn are multipolar, in cross-sections the 

 cell-bodies appearing irregularly polygonal and in longitudinal sections fusiform in out- 



1 Anatomischer Anzeiger, Bd. xl., 1912. 



