THE NERVOUS SYSTEM 296 



medullary mass contained within the cranial cavity. The two 

 hemispheres, however, which are confused with the brain, should be 

 distinguished from it ; because they form together a special organ 

 added on to the brain, have special functions of their own and do not 

 contain the centre of communication of the sensitive system. 



Although the true brain, that is to say, the medullary part which 

 contains the nucleus of sensations and to which the nerves lead from 

 the special senses, is difficult to identify and define in man and intelhgent 

 animals, on account of the contiguity or union between this brain and 

 the two hemispheres which cover it, it is none the less true that these 

 hemispheres constitute an organ specially related to the functions that 

 it performs. 



Indeed it is not in the brain properly so-called that ideas, judgments, 

 thoughts, etc., are formed ; but it is in the organ superimposed on it, 

 consisting of the two hemispheres. 



Nor is it in the hemispheres that sensations are produced ; they 

 liave no share in it, and the sensitive system exists satisfactorily 

 without them ; these organs may therefore undergo great degeneration 

 without any injury to feeling or hfe. 



I now revert to the general principles concerning the composition 

 of the various parts of the nervous system. 



The nervous threads and cords, the gangUonic longitudinal cord, 

 the spinal cord, the medulla oblongata, the cerebellum, the cerebrum 

 and its hemispheres ; — all these parts have, as I have already observed, 

 a membranous and aponeurotic investment which serves as a sheath, 

 and which by its pecuhar nature retains within the medullary sub- 

 stance the special fluid that moves about there ; but at the extremities 

 of the nerves where they terminate in the parts of the body, these 

 sheaths are open and allow the nervous fluid to communicate with 

 the parts. 



Details about the number, shape and situation of the parts I have 

 referred to, belong to the sphere of anatomy ; an exact description 

 may be found in works which deal with this sphere of our knowledge. 

 Now since my purpose here is simply to investigate the general prin- 

 ciples and faculties of the nervous system, and to enquire how nature 

 first conferred it on such animals as possess it, I need not enter into any 

 of the details that are known about the parts of this system. 



Formation of the Nervous System. 



We certainly cannot positively determine the manner in which 

 nature brought the nervous system into existence ; but it is quite 

 possible to ascertain the conditions which were necessary for this 

 purpose. When once we have ascertained and studied these con- 



