298 ZOOLOGICAL PHILOSOPHY 



poured into that medullary substance which is so well adapted to 

 contain it. 



Lastly, I shall conclude these reflections by some remarks on the 

 development of the main medullary mass and of the swelHngs and 

 expansions which are found in certain parts of that mass. These 

 expansions are proportional to the formation and development of the 

 special systems which compose the common and perfected nervous 

 system. 



In the main medullary mass of every nervous system, the particular 

 part which to some extent gave origin to the rest, need not necessarily 

 be larger than the other parts which have grown from it ; for the 

 thickness and size of these other parts are always dependent on the use 

 which the animal makes of the nerves that issue from them. I have 

 given sufficient proof of this in the case of all the other organs : the 

 more they are exercised the more they become developed, strengthened, 

 and enlarged. It is because this law of animal organisation has not 

 been recognised, or because no attention had been paid to it, that 

 it is beheved that the part of the medullary mass which produces the 

 other parts must of necessity be larger than them. 



In vertebrates, the main medullary mass consists of the bram and 

 its accessories, the medulla oblongata and the spinal cord. Now it 

 appears that the part of this mass which produced the rest is really 

 the medulla oblongata ; for it is from this part that issue the medullary 

 ■appendages (the peduncles and crura) of the cerebellum and cerebrum, 

 the spinal cord, and, lastly, the nerves of the special senses. Yet the 

 medulla oblongata is in general smaller or less thick than the brain 

 which it has produced, or the spinal cord which proceeds from it. 



Whereas, on the one hand, the brain and its hemispheres are employed 

 in acts of feeUng and intellect, while the spinal cord only serves for the 

 excitation of muscular movements ^ and the performance of organic 

 functions ; and whereas, on the other hand, the continued use or exercise 

 of the organs causes in them a remarkable development ; it must 

 follow that in man, who is continually exerting his senses and intellect, 

 the brain and hemispheres should become much enlarged while the 

 spinal cord, which in general is little used, can only acquire moderate 

 dimensions. Finally, since the chief muscular movements of man are 

 those of the arms and legs, we should expect to find a conspicuous 

 swelhng in his spinal cord at the exits of the crural and brachial 

 nerves, and this is confirmed by observation. 



In those vertebrates, on the contrary, which make but httle use of 



^ With regard to the function of the spinal cord in providing nervous influence 

 to the organs of movement, recent experiments have shown that poisons which act 

 on this cord do actually cause convulsions and attacks of tetanus before producing 

 death. 



