120 THE MECHANISM OF LIFE 



Now add to all ttis (1) a new series of connections between 

 the nerve cells of the grey matter in segment (6) and the cere- 

 bellum, and note that there must be two sets of paths, to and 

 from the cerebellum, included in these connections. Add (2) a 

 series of connections between the segment (a) and the mid-brain, 

 also to and from paths ; these are, of course, the tracts described 

 on pp. 99-104 and in Figs. 28 and 31. Add, finally, (3), the con- 

 nections between the mid-brain and the cortex on the one hand, 

 and the mid-brain and the cerebellum on the other- — these also 

 being to and from path& — and the reader may get some idea of 

 the complexity of the machinery of nerve cells and nerve tracts 

 .that are involved in a relatively simple action when it is the 

 object of control by will or experience. 



The " Simple " Reflex Mechanism. 



The schematic simple reflex mechanism, such as it is figured 

 in p. 117, is to be regarded as a " fiction" in the conventional, 

 legal sense — that is to say, it is a " scheme " which is useful in 

 enabling one to understand the maze of nervous tracts and paths 

 along which impulses travel within the nervous system. If we 

 could isolate all the afferent fibres connecting one group of 

 nerve cells in the spinal cord with one group of receptor organs 

 in the skin, these would represent the afferent path of a simple 

 reflex. Then we should have to isolate all the nerve fibres 

 connecting the same group of nerve cells with some one muscle, 

 and that would be the efferent path. So we should obtain our 

 schematic " reflex arc." 



Nothing like this exists in the higher animal. The afferent 

 impulses originating in any one small group of receptors do not 

 go to one segment only in the spinal cord, but to several such. 

 Each of these segments is connected with the same muscle, for 

 the fibres going out from the cord through one motor root and 

 ending in one muscle are derived from several segments. Thus 

 one efferent and one peripheral afferent path are connected 

 together by various nuclei, and these nuclei are joined by several 

 intraspinal paths. 



Then we have the further complications indicated in Fig. 34. 

 The schematic single efferent paths are really double ones, each 

 of them including a nerve branch going to a muscle and to its 

 antagonist. The same impulse which breaks upon the synapse 

 in the nucleus stimulates the nerve going to the muscle, so that 



