MINUTE STRUCTURE OF SPINAL CORD. 177 



that one or more of these fibrils is continued to form the 

 axis cylinder of a nerve-fibre in a dorsal root of one of the 

 spinal nerves. 



As we shall learn later, the dorsal roots are concerned in 

 carrying impulses from the outer world to the spinal cord; 

 the anterior roots in conveying impulses from the nerve- 

 centres to the organs (muscles, glands, etc.) of the body. 

 Therefore, in general terms, we may speak of the type of 

 nerve-cell 1, Fig. 73, as a motor nerve-cell; and the type 

 of cell 2, Fig. 73, as a sensory nerve-cell. Both varieties 

 of cells are found in the gray matter of the brain. 



In the sympathetic and sporadic ganglia somewhat 

 simpler forms of nerve-cells occur. 



Nerve-Centres consist of white and gray nerve-fibres, of 

 nerve-cells, and of connective tissue and blood-vessels ar- 

 ranged in different ways in the different centres. Ganglia 

 are collections of nerve- cells and nerve-fibres, some of the 

 latter being connected with the cells, while others seem 

 merely to pass through the ganglion on their way to other 

 parts. As an illustration of the structure of a more com- 

 plex nerve-centre we may study the spinal cord. 



Histology of the Spinal Cord. If a thin transverse sec- 

 tion of the spinal cord be examined with a microscope it 

 will be found that enveloping the whole is a delicate 

 layer of connective tissue, the pia mater. Fine bands of 

 it ramify through the cord, supporting the nervous ele- 

 ments; some of the coarser of these are represented at 6, 7, 

 and elsewhere in Fig. 74; but from these still finer proc- 

 esses arise, as represented at d and e in Fig. 75. The 

 ultimate finest tissue supporting the nervous elements di- 

 rectly, is the neuroglia (p. 106). In the white columns, the 

 cord (Fig. 75) will be seen to 'be mainly made up of medul- 

 lated nerve-fibres which run longitudinally and therefore 

 appear in the transverse section as circles, with a dot in 

 the centre, which is the axis cylinder. At 1) in Fig. 75 

 these fibres are represented, the intermediate connective 

 tissue being omitted, while at e this latter alone is repre- 

 sented in order to show more clearly its arrangement. At 



