112 HUMAN PHYSIOLOGY 



with a hot surface it is instantly withdrawn. Here 

 the incoming or afferent message starts the action 

 which involves the conveyance of the sensation of 

 heat to the centres governing the muscles of the 

 arms, which operate to withdraw the limb. For the 

 performance of a reflex action, we therefore find 

 that what is required, apart from more intricate 

 details, are essentially an incoming nerve fibre, an 

 outgoing nerve fibre, and a nerve cell or centre 

 which deals with and reflects the message it receives. 

 A reflex action may, of course, be performed 

 unconsciously and without involving the arousing of 

 attention. An act, such as that of closing the eye- 

 lids in face of danger, for example, takes place auto- 

 matically, and is all the more quickly performed on 

 that account. 



THE SPINAL CORD. Confining our attention to the 

 brain system, we find this portion of the nervous 

 apparatus (Fig. 30) to include (1) the brain itself 

 contained and protected within the skull, (2) the 

 spinal cord issuing from the brain and representing 

 the main line of the nervous system contained within 

 the spine, and (3) the various nerves given off from 

 brain and spinal cord together. The spinal cord, it 

 should be noted, is much more than a great line of 

 nerve fibres. It contains both nerve cells and fibres, 

 and when we make a section of the cord we find 

 its cells to be contained in what is called the grey 

 matter in the centre of the cord, the white matter 

 outside, representing the nerve fibres which place 

 brain and body in connection. Thirty-one pairs of 

 nerves (Fig. 30) are given off from the spinal cord. 

 One of the most interesting discoveries noted in 

 connection with the nervous system was elicited 

 when the reason for each spinal nerve being 



