148 THE MECHANISM OP LIFE 



Now, but for two exceptional paths — that of the impulses 

 passing directly into the cerebral hemispheres through the 

 olfactory nerves, and that of the impulses going to the cerebellum 

 from the vestibular part of the auditory organ — the parts repre- 

 sented above the line CD in Fig. 45 connect together the various 

 components of the central nervous system — they are the integrating 

 mechanisms. 



Thus the cerebellum receives impulses directly from the 

 muscles and joints of the body and others indirectly {via the mid- 

 brain) from the skin; it receives impulses indirectly (also via the 

 mid-brain) from the organs of special sense in the head, and it is 

 in direct (to and from) communication with the cortex. And so 

 it has a grip on all the afferent impulses, whether those of general 

 or special sensation, arising anywhere in the body. 



The cerebral cortex has conspicuous connections with the 

 cerebellum, as we have just said, but the communications with 

 the nuclei of the mid-brain, in which the nerves of sense end, 

 are just as prominent, and the great pyramidal tract leading down 

 from the motor region of the cortex to the nuclei of the motor 

 spinal nerves is more conspicuous still. And so the grip of the 

 cortex cerebri on both receptor and effector organs is as evident 

 as that of the cerebellum on the receptor organs. 



We may now resume the general working of this whole 

 mechanism. The parts below the line AB constitute relatively 

 simple reflex arcs sufficient in themselves for movements initiated 

 by the stimulation of the sense organs in the limbs and trunk. 

 The parts between AB and CD — that is, the mid-brain complex — 

 also form reflex arcs, but the distance receptors (visual and 

 auditory organs) are now included, and become incorporated 

 with the spinal arcs by means of the tracts joining mid-brain and 

 cord. The stimuli initiating movements are now much more 

 varied and numerous, and so the latter become more complex 

 than when they depend on the cord alone. 



Finally, when the parts above CD are fully evolved, something 

 quite new is added to the activities of the central nervous system. 

 The cerebellum mediates between the impulses coming from the 

 receptors in general (but particularly those in the muscles and 

 joints), and the impulses that pass out from the cortex to set 

 muscular mechanisms in action. It is not concerned in the 

 psychical life of the animal to any degree that can be recognised, 

 but it " standardises " and co-ordinates a number of very com- 



