318 



PHYSIOLOGY 



group often subdivided 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 irregu- 

 larly through the grey matter, especially of the posterior horn. 



M lot. 

 asc.trat. 



Direct 

 Cerebellar 



Posiei'ior Roots, 

 wiik collaterals. 



FIG. 155. Spinal cord. (After LENHOSSEK.) On left side of figure are shown 

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

 tribution 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. 



According to the destiny of their axons these nerve-cells may be divided into four 

 groups (Fig. 155). 



(1) THE MOTOR CELLS, the largest of all, which send their axons into the anterior 

 roots, where they run to supply skeletal muscle fibres. A t a sub-group of these cells we 

 may class the somewhat smaller cells of the lateral horn, which in all probability send 

 their axons by the anterior roots to supply visceral muscles. Their axons can be 

 distinguished from the motor axons by the smaller diameter of the nerve fibres they 

 form. They pass later from the mixed nerve along a white ramus communicans into 

 the sympathetic system, in the ganglia of which they end. 



(2) CELLS OF THE COLUMNS. As typical of these cells we may take those 

 which form Clarke's column. Their axons do not leave the central nervous system, 

 but pass out into the white matter to some other part of the central nervous system, 

 contributing thus to form the white columns of the cord. 



(3) COMMISSURAL CELLS. These cells send their axon across the middle 

 line to the opposite side of the cord, making up a great part of the anterior white com- 

 missure. 



(4) CELLS OF GOLGI. These cells are found chiefly in the posterior horn. They 

 are multi-polar and are distinguished from all the other cells by the fact that their 



