14 THE SPINAL CORD. 



the central canal and ventricles of the brain extend a long and indefinite distance into 

 the grey matter. It is probable however that it is by transverse division of the 

 spongioblasts, preceded by division of their nuclei, that the neuroglia-cells are origin- 

 ally formed. At the same time it appears clear that although nerve-cells and 

 neuroglia-cells are functionally very different, their origin is not so dissimilar as was 

 at one time believed to be the case. It was taught formerly that the neuroglia is a 

 form of connective tissue (which it resembles functionally), and it was supposed that 

 its cells were developed from meso blast, but of late years there has been much doubt 

 cast upon its supposed mesoblastic origin, and the question is not yet fully decided. 

 The researches of His have tended however to show that both the spongioblasts 

 from which the neuroglia-cells are believed to originate and the neuroblasts which 

 give origin to the nerve-cells are both formed from the (at first undifferentiated) 

 cells of the neural epiblast (see Embryology, development of central nervous system). 

 The investigations of Ramon y Cajal upon the development of these elements in the 

 embryo chick and mammal further indicate that the distinction into spongioblasts 

 and neuroblasts is not a fundamental one, for cells which from their shape and 

 position would be classed amongst the spongioblasts of His may alter their character 

 and by throwing out an axis-cylinder process become transformed into nerve-cells. 



Small concentrically striated globules, termed corpora amylacca, are frequently met with 

 in the neuroglia of the cord in man, as well as in many of the parts of the central nervous axis. 

 They appear to be composed of proteid substance but, although long recognized, their mode 

 of formation and their meaning are unknown. 



DISTRIBUTION OF NERVE-CELLS IN THE SPINAL CORD. 



In transverse sections of the cord it is seen that the nerve-cells are not equally 

 distributed throughout the grey substance, but are arranged in definite groups, 

 which occupy nearly the same relative position in successive sections. The groups 

 are therefore the sections of longitudinal tracts of grey matter rich in nerve- 

 cells, and these tracts are named the gangUonic or cell-columns of the grey matter. 

 The longitudinal continuity of the groups can be seen in sections of the cord 

 made parallel with its long axis and passing through the part of the grey matter 

 where the groups occur. In such longitudinal sections it may also be observed that 

 the cells tend, speaking generally, to be more extended parallel to the longitudinal 

 axis of the cord the longer the segments of the cord, as indicated by the entering 

 nerve-roots (Toldt). It may further be stated as a probable law, applicable at least 

 to the cells belonging to the same group or column, that the longer the nerve-fibre 

 which issues from a cell, the larger is the cell. Those segments of the cord 

 from which the longest nerve-fibres issue by the anterior root have the largest 

 anterior horn cells (Pierret). 



Cell-column of anterior horn. Of these groups or columnar tracts of nerve- 

 cells, the one which is most constant and contains the largest cells is found along 

 the whole of the ventral part of the anterior horn where the nerve-cells lie among 

 the issuing fibres of the anterior roots. There seems to be no doubt that many of 

 these anterior or motor nerve-fibres are directly continuous with the axis-cylinder 

 processes of nerve-cells of this group. Hence it is sometimes named the motor 

 cell-column, but it is more generally known as the cell-column of the anterior horn. 

 Its cells are in most parts collected into two groups, a lateral or ventro-lateral 

 (fig. 14, 1) nearer the lateral column of white matter, and a mesial (a) nearer the 

 anterior column ; in the cervical and lumbar enlargements there is a third, more 

 deeply seated, dorsal or dor so-lateral group (&'). 



Of these several groups of cells into which 1 the anterior horn cell-column is 

 divisible, the one which is most constant along the whole length of the cord is the 



