THE NERVOUS SYSTEM 293 



The threads and cords of which I spoke above are nerves ; and the 

 main medullary mass, which contains the centre of communication of 

 the system, consists in some invertebrates either of separate ganglia 

 or of a ganglionic longitudinal cord ; in the vertebrates, it forms the 

 spinal cord and the medulla oblongata which is united to the brain. 



Wherever a nervous system exists, however simple or imperfect it 

 may be, there is always a main medullary mass in some form or other ; 

 for it constitutes the basis of the system and is essential to it. 



It is in vain to deny this truth by such arguments as the following : 



1. That it is possible to remove entirely the brain of a tortoise or 

 a frog, which nevertheless continue to exhibit movements showing 

 that they still have sensations and a will : I reply, that this operation 

 only destroys a part of the main medullary mass, and not that part 

 which contains the centre of communication or sensorium commune ; 

 for this is not contained in the two hemispheres which form the bulk 

 of what is called the brain ; 



2. " That there are insects and worms, which when cut into two 

 or more pieces, promptly form so many new individuals, each having 

 its own system of sensation and its own will " ; I reply again, that as 

 regards insects the alleged fact is untrue, that no experiment has shown 

 that when an insect is cut in two there may result two individuals 

 both capable of life ; and even if it were so, each half of the insect 

 would still possess a main medullary mass in its share of the ganglionic 

 longitudinal cord ; 



3. " That the more evenly the nervous substance is distributed, 

 the less essential is the rôle of the central parts." ^ I reply, for the last 

 time, that this assertion is erroneous ; that it has no facts to support 

 it ; and that it is only made through ignorance of the functions of the 

 nervous system. Sensibility is neither the property of nervous sub- 

 stance nor any other substance, and the nervous system can only 

 enter upon its functional existence when it is composed of a main 

 medullary mass from which nervous threads take rise. 



Not only can the nervous system have no functional existence 

 unless it is composed of a main medullary mass which contains one or 

 more nuclei for starting muscular excitement and from which various 

 nerves proceed to the parts, but we shall also see in Chapter III. that 

 the faculty of feeling in any animal can only arise when the medullary 

 mass contains a single nucleus or centre of communication, to which 

 the nerves of the sensitive system travel from all parts of the body. 



It is true that the extreme difiSculty of following these nerves to their 

 centre of communication, has led some anatomists to deny the existence 



1 See L' Anatomie comparée of M. Cuvier, vol. ii., p. 94 ; and the Recherches sur le. 

 Système nerveux of MM. Gall and Spurzheim, p. 22. 



