MICROSCOPIC STRUCTURE. :17 



tract (lateral cell-column, fig. 14, d). Like that tract, the ganglionic column it 

 contains chiefly appears as a distinct formation in the thoracic region ; in other parts 

 the cells do not form so distinct a group, but they appear nevertheless to be repre- 

 sented to some extent along the whole cord. Its cells are bipolar or multipolar, and 

 measure on an average 80/z in diameter, being considerably smaller than those of 

 Clarke's column or those of the anterior horn. In the upper part of the cervical 

 region a group of cells becomes distinct in a similar situation (lateral nucleus), and 

 is traversed by the roots of the spinal accessory nerve. 



Middle cell-column (Waldeyer). This is formed of what in sections of the 

 cord forms an ill-defined group of moderately-sized cells placed in the middle of the 

 grey matter of the crescent. The cells are most distinct as a group in the cervical 

 region, and also more numerous there, becoming fewer in number as Clarke's column 

 becomes more evident. In the dorsal region they lie mostly at the side of Clarke's 

 column, but further down they again move nearer the middle of the crescent. They 

 are distinguishable right down into the sacral region. It is not known with what 

 fibres they are connected, but fibres from the postero-lateral columns course amongst 

 them, and it may be that, on the other hand, they give off nerve-processes to one of 

 the conducting tracts ( ? to the antero-lateral ascending tract). They stain less 

 deeply with carmine than those of the motor column or of Clarke's column. 



Cells of posterior horn ; solitary cells. The cells of the posterior horn 

 are not grouped very definitely, but for purposes of description they may be roughly 

 sub-divided into 1, those at the base of the horn ; 2, those near the middle of the 

 horn ; 3, those at the margins ; and 4, those of the gelatinous substance of Rolando. 

 They vary in size, some of the largest being found near the mesial margin of the 

 horn ; these often have a characteristic long curved process (comet-cell, Waldeyer) : 

 and even within the same group both large and small cells may be found intermixed. 



But in addition to the groups, a number of scattered cells are met with, dis- 

 tributed through the posterior horn. These cells vary much in form and size, but 

 are for the most part spindle-shaped. They are usually spoken of as the solitary 

 cells. The axis-cylinder processes both from some of these cells and from cells of the 

 lateral cell-column pass towards the anterior horn and also towards the anterior 

 commissure, and they are believed to give origin to the smaller fibres which issue 

 with the anterior roots. Some of the axis-cylinder processes of these cells do not 

 however leave the grey matter but are branched and their ramifications lose them- 

 selves in the interlacement of fibrils which invests other cells. In the lamprey it has 

 been shown by Freud that cells which appear to correspond with the solitary cells 

 send their axis-cylinders into the posterior roots, and more than one observer has 

 described a direct passage of the axis-cylinder process of one of the more deeply 

 lying cells of the anterior horn through the grey matter into the posterior root, and 

 thence past the ganglion into the mixed nerve without coming into connexion 

 with any of the ganglion-cells. 



The cells of the substance of Rolando were first noticed by Gierke. They are 

 small round granule-like cells, closely packed and staining with difficulty. They 

 appear to have been often taken for neuroglia cells, but, according to Gierke and 

 H. Virchow, there is little doubt about their nervous nature. 



Cells are occasionally found separated from the general mass of grey matter and 

 lying out amongst the fibres of the white columns. These " outlying cells " have 

 been described by Stilling and others, recently in detail by Sherrington. 



It has been shown by Golgi that the nerve-cells of the central nervous system may be 

 primarily classified under two heads, viz., 1, those with a long axis-cylinder process which 

 becomes a medullated nerve-fibre ; and 2, those with a short axis-cylinder process which may 

 or may not become medullated, and soon ramifies and loses itself in neighbouring parts of 

 the grey matter. Golgi is of opinion that the former are to be looked upon as " motor or 



VOL. III. C 



