THE SPECIAL NERVOUS MECHANISMS 121 



the latter contracts, and it also stimulates the nerve going to the 

 antagonist, so that it relaxes. These two antagonistic effects 

 must be regarded as the single functional muscular action' — 

 that is, their effect is, say, to bend or extend a limb. 



But the movement of the limb is something that varies to 

 a very great degree: the part may be completely or partially 

 bent (or extended), and it may bend or extend against resistances 

 which also vary immensely. The duration of the movement 

 therefore varies, and so also does the quantity of energy trans- 

 formed — more or less work is done according to the circum- 

 stances under which the action is performed. To some extent 

 all this regulation is carried out in the nucleus from which the 

 efferent impulses proceed out to the muscle, but it is also the 

 work of the contracting (or relaxing) muscle itself. There are 

 receptors in the latter which are affected or stimulated by the 

 events taking place in it, and by the circumstances of the action 

 — the load, for instance, borne by the contracting muscle. These 

 receptors generate impulses which ascend into the nucleus and 

 modify, if need be, the efferent impulses sent out by the latter. 



A mechanical analogy may make this very important function 

 of the muscle frofrioceftors clear. Let us think about a small 

 railway system in which the movements of every train are 

 initiated and regulated by operators working in a central 

 telegraphic control station. But as every train proceeds on 

 its journey, moves from block to block, stops at and starts from 

 a station, it records its position telegraphically (and perhaps auto- 

 matically) on a time and space chart in the control office, so that 

 the operator may see at each movement where it actually is. That 

 would represent the system of efferent nerves going from the 

 spinal cord nucleus to the muscular apparatus, and the receptors 

 of the latter, with their afferent nerves, going back into the nucleus. 



These, then, are the mechanisms concerned in the simplest 

 spinal reflex arc: several cord segments in receipt of afferent 

 impulses from the skin, and all of them in connection with the 

 same small group of antagonistic muscles and a system of re- 

 ceptors in those muscles in connection with the nuclei from 

 which the efferent impulses start. 



The Cranio-Spinal Reflexes. 



The muscular apparatus concerned in movements are, we 

 have seen, under the immediate control of ganglia or nuclei in the 

 spinal cord, or (what are very similar) the ganglia in the medulla 



