12 



centres are eitlier fused together or connected with each 

 other by bands. The system may be roughly described 

 as consisting of ingoing and outgoing nerves, and a 

 central system of connected ganglia — the spinal cord and 

 brain. All the ordinary nerves enter the cord. The nerves 

 of the special senses enter the brain. The spinal cord con- 

 sists of nerve-centres surrounded by nerve-fibres ascending 

 to or descending from the brain. 



The ascending or sensory fibres cross to ojjposite sides 

 in the cord. The descending or motor fibres cross ahove 

 the cord. The upper part of the cord is called the medtilla. 

 Above the medulla and in the brain- sub stance are two 

 centres, one on each side, into which the ascending or 

 sensory fibres of the cord pass as well as the ingoing 

 nerves of the special senses. These are known as the 

 optic tlmlami. Close by there are two other centres, one for 

 each side, from which the descending or motor nerves 

 take their origin. These are called the corpora striata. 

 The optic thalamus of each side is connected by nerve- 

 bands with its fellow on the opposite side, and with the 

 corjius striatum of its own side. Above and around these 

 centres is the great system of centres forming the surface 

 of the brain, the cortex. Each half of the cortex is con- 

 nected with its fellow of the opposite side, and every part 

 with every other part by an infinite number of interlacing 

 fibres. Ascending fibres reach it from the optic thai ami 

 and descending fibres pass from it to the corpora striata. 



There are other centres of the brain, with which, in 

 order to keep the subject as little complicated as may be, 

 I shall not deal this evening. I have sufficiently demon- 

 strated what I may venture to call the mail-route and 

 chief sorting stations of the nervous system. 



Let us inquire into the functions of these various parts. 



When, as a result of injury, the spinal cord is divided 

 at any j^oint, all the parts supplied by nerves below the 

 seat of injury become paralysed. Beyond that point the 

 sufferer can neither move nor feel. But if you tickle the 

 sole of the paralysed foot you will ehcit a convulsive 

 movement which is neither felt nor controlled ; this is called 

 reflex action. It is exactly what happens if you tickle the 

 soles of a sleeping person. It is what happens when 

 your finger is grasped by a sleeping child. But while the 

 brain remains connected with the cord, a person may to 

 some extent restrain reflex action. 



