358 



PHYSIOLOGY 



cells provided with a very large number of branches and representing 

 the cells from which all the fibres of the neuroglia have been derived. 

 The neuroglia is present in specially large amount in two situations, 

 namely, immediately around the central canal and as a capsule to the 

 enlargement of the posterior cornu, known as the head or caput cornu 

 posterioris. In this latter situation the neuroglia contains a large 

 number of small richly branched nerve-cells and is spoken of as the 

 substantia gelatinosa of Rolando. The nerve-cells are arranged in 

 distinct groups. In the anterior horn we may distinguish three 

 groups ; a median group of cells near the middle line, many of which 



Direct 

 Cerebellar 



FIG. 156. Spinal cord. (After LENHOSSEK.) On left side of figure are shown 

 the nerve-cells with their axis-cylinder processes. On the right side the 

 distribution of the chief collaterals. 



1, motor cells ; 2, cells of the columns ; 2a, cells of Clarke's column, sending 

 processes across into direct cerebellar tract ; 3, 4, and 5, commissural cells. 



send their processes across to the other side in the anterior white cornu. 

 and an external group often subdiyided into a postero- external and 

 an antero- external. The latter group is especially developed in the 

 regions of the cervical and lumbar enlargements and consists of very 

 large multipolar cells with many dendrites which send their axons 

 into the anterior roots and by these to the muscles of the limbs. 

 Another group of rather smaller cells is found in the lateral horn, in 

 that region of the cord where this is marked. A very definite group 

 of cells may be seen in the dorsal region of the cord in the inner aspect of 

 the root of the posterior horn. This, which is known as Clarke's 

 column, is formed by large cells elongated in the longitudinal direction 

 of the cord. Besides these definite columns a number of nerve-cells 

 are distributed irregularly through the grey matter, especially of the 

 posterior horn. According to the destiny of their axons these nerve- 

 cells may be divided into four groups (Fig. 156). 



