116 THE MECHANISM OF LIFE 



mid-brain as the two optic tracts, wliich then partially cross 

 each other in the middle line of the head. The further course of 

 the optic tracts in the brain is now fairly well known. 



Fig. 32 represents these tracts in a very schematic way. Some 

 of the fibres of the right optic nerve pass over to the left-hand 

 side of the brain, while some keep on the same side (and vice 

 versa). Then all the optic fibres terminate by forming synapses 

 with the cells in three nuclei contained in the optic thai ami and 

 corpora quadrigemina. That is their lower-brain termination, 

 but a very obvious tract of other fibres, the optic radiations, 

 start ofE as the axons of cells in the lower visual centres or nuclei, 

 and proceed up to the cortex cerebri. 



Audition and vision are complex sensations. When we hear 

 we distinguish loudness (that is, the amplitude or " intensity " 

 of the sound waves), pitch (which is the frequency of occurrence 

 of the sound waves), and musical quality (which we explain by 

 assuming that the sound waves are complex, and can be de- 

 composed into components). Similarly, in vision we distinguish 

 between intensity of light and quality of light (or colour). By 

 intensity we mean the amplitude of the vibrations of the medium 

 (ether) which transmits that which we recognise as light, and 

 by colour we mean the components of this mixed light as they 

 are separated from each other by the physical media outside the 

 retina itself. How the analyses of sound and visual stimuli are 

 carried out by the auditory and visual organs is, of course, far 

 from being understood, and we cannot discuss the question now. 



So much, then, for a very summary consideration of the ways 

 by which the stimuli of the receptor organs of the body are 

 transmitted into the central nervous system and brought to 

 bear upon the various ganglia there. We must next consider — 



The Motor Mechanisms. 



From what has been said in Chapter II. the reader will already 

 know that a motor mechanism includes (1) an afferent nervous 

 path leading into the central nervous system; (2) a nucleus or 

 ganglion; and (3) an efferent nervous path leading out from the 

 ganglion to the motor organ. Thus we take the case of a purely 

 special mechanism. 



Some receptor organ is stimulated (say a touch spot in the 

 skin), and an afferent impulse is set up and propagated along a 

 spinal nerve into the grey matter of the cord. This impulse 



