28 THE MECHANISM OF LIFE 



Fig. 9 shows, the two series of dendrites do not touch each other, 

 but the branches of the arborisations " interdigitate." They 

 approach each other very closely, but there is always a space 

 between them, and this space is, of course, filled by other tissue 

 or by liquids. This mode of joining up of successive neurones 

 to form a chain is universal throughout the nervous system, 

 both in the central and the peripheral regions. 



An Example of Sensori-Motor Activity. We shall take what 

 is, perhaps, the simplest and most convenient common action, 

 that of " winking." When something unexpectedly and rapidly 

 approaches the face, the eyes " instinctively " close in order 

 that they may be protected. Now this action of closing and 

 subsequent reopening involves a receptor organ, an afferent 



To cortex. 



SgQSfL 



otor 

 Centres oculomotor 



Jrorn cortex 



Closing muscle- -3 



FIG. 10. SCHEME OF THE NEURONES CONNECTING THE RETINA WITH 

 THE MUSCLES OF THE EYELIDS. 



nerve, a nerve centre, an efferent nerve, and a motor organ. 

 The receptor is the retina of the eye. Light reflected from the 

 moving object forms a minute picture on the retina, and in some 

 way the proximal dendrites (which in this case, however, are not 

 dendrites in the usual sense, but rather the rods and cones of the 

 retina) are affected by the variations in light and shade (that is, 

 by differences in the intensity and quality of the radiation), and 

 nervous impulses are set up which travel along the axons of the 

 retinal nerve cells in the optic nerve to the brain. There they 

 are received by a nerve centre, and the afferent impulse is con- 

 verted into an efferent one, which travels out from the centre, 

 via the axons of the nerve cells there, through two nerves. One 

 of these is the third cranial, or oculo-motor nerve, and the other 

 is a branch of the seventh cranial, or facial nerve. The former 



