STRUCTURE OF THE HUMAN BODY 



SI 



Fig. 28. — Various Forms of Nerve-cells, b and d frum 

 ganglia on dorsal roots of spinal nerves. Observe nucleus 

 in centre of each. 



meter. The grey matter is mainly made up of angular or 

 star-shaped structures of very small size, each of these being a 

 nerve- or ganglion-cell (fig. 28). It is these cells which collec- 

 tively make up the essential part of the central organs, while 

 the white matter and the nerves consist of an infinite number 

 of slender nerve-fibres which have 

 a conducting function. To take 

 an electrical analogy, the nerve- 

 cells may be looked upon as 

 batteries, while the nerve-fibres 

 are comparable to wires. The 

 analogy, however, is a very rough 

 one, for here batteries and wires 

 alike are alive, and there does not 

 appear to be the same coupling 

 up of nerve cells and fibres that 

 exists between batteries and wires 



in an ordinary electric system. There is indeed a certain amount 

 of connection, but how much is a matter of doubt. Not impro- 

 bably the cells, to some extent, are able to act independently of 

 fibres, recalling the wireless telegraphy which is now being 

 developed. 



Subject to the brain, a large amount of control over the neck, 

 trunk, and limbs is exerted by the spinal cord, and in accordance 

 with this the parts in question are supplied by numerous pairs of 

 spinal nerves which arise from the cord. The examination of any 

 particular spinal nerve (fig. 27) will show that it takes origin in 

 a couple of bundles of fibres which, from their position, are re- 

 spectively known as the dorsal and ventral roots. It has been 

 shown that, broadly speaking, the fibres of the dorsal root are 

 carrying nerve impulses — telegraphic messages, as it were — to 

 the central organs, while those of the ventral root carry such 

 impulses away from them. To express this, the names af- 

 ferent and efferent fibres (L. affero, I carry to; effero, I carry 

 out) are respectively given. The afferent fibres convey infor- 

 mation to the central organs about what is going on in the 

 body or outside it, while the efferent fibres carry impulses from 

 the central organs to the other parts of the body, with the 

 result that certain actions take place, or it may be that actions 

 in progress are modified or stopped. 



