1885.] on the Motor Centres of the Brain, dc. 251 



s\'stem, is due to the fact tliat miro volition is dcjicndent entirely on 

 the exercise of the attention which connotes the idea of singleness. 

 Consequently that it is impossible to carry out two totally distinct 

 ideas at one and the same moment of time, when the attention must 

 of course be fully engaged upon each. 



I fear that in making my argument consecutive, T shall have to 

 pass over very well-beaten paths, and so I must ask your patience for 

 a few moments while I make good my premisses. 



The nervous system, which in man is com^iosed of brain, spinal 

 cord, nerves, and nerve-endings, is arranged upon the simplest plan, 

 although the details of the same become highly complex when we 

 arrive at the top of the brain. 



At the same time, while we have this simple plan of structure, we 

 find that there is also a fundamental mode of action of the same — a 

 mode which is a simple exposition of the principle, no effect without 

 a cause — a mode of action which is known as the phenomenon of 

 simjile reflex action. 



The general plan of the whole nervous system is illustrated by 

 this model. Imbedded in the tissues all over the body, or highly 

 specialised and grouped together in separate organs, such as the eye 

 or ear, we find large numbers of nerve-endings, that is, small lumps 

 of protoplasm from which a nerve fibre leads away to the spinal cord 

 and so ujd to the brain. 



These nerve-endings are designed for the reception of the different 

 kinds of vibration by which energy presents itself to us. As the 

 largest example of these nerve-endings, let me here show you one of 

 the so-called Pacinian bodies, or more correctly Marshall's corpuscles, 

 for Mr. John Marshall discovered these bodies in England before 

 Pacini published his observations in Italy. Here you see one of 

 these small oval bodies arranged on the ends of one of the nerves of 

 the fingers, and here you see the nerve fibre ending in the little 

 protoplasmic bulb which is protected by a number of concentric 

 sheaths. 



Pressure or any form of irritation of this body at the end of the 

 nerve fibre causes a stream of nerve energy to travel through the 

 spinal cord to the brain, and so we become conscious that something 

 is happening to the finger. 



Here in this section of the sensitive membrane of the back of the 

 eye, the retina, you see a similar arrangement, only more complicated, 

 namely, nerve fibres leading away from small protoplasmic masses 

 which possess the property of absorbing light and transforming it 

 into nerve energy. It is this transformation into nerve energy of 

 heat, light, pressure, &c., which it seems to me should alone be called 

 a sensation, irrespective of consciousness. And in fact we habitually 

 say we feel a sensation. The terms feeling and sensation, however, 

 are frequently used as interchangeable expressions, although, as I 

 shall show you directly, "feeling " is the conscious disturbance of a 

 sensory centre in the surface of the brain, and in fact feeling is the 



