34» INTRODUCTION. 



movements discernible upon the face of nature. 

 There is further every probability that this is so, 

 inasmuch as it is clearly demonstrated that the im- 

 petus of the nerve on the fibre is not mechanical. 



The medullary substance of the whole nervous 

 system is homogeneous, and must, wherever it is 

 found, be in a capacity to exercise the functions 

 which appertain to its nature. An abundance of 

 the blood-vessels are discernible in all its ramifi- 

 cations. 



As all the other animal fluids are derived from 

 the blood by secretion, there is every reason to pre- 

 sume that the nervous fluid is in the same situation, 

 and that the medullary substance is the agent of its 

 secretion. 



On the other hand, it is most certain that the 

 medullary substance is the only conductor for the 

 nervous fluid. All the other organic elements act 

 as repellants of this fluid, and serve to arrest its 

 progress, as glass does that of electricity. The 

 external causes which are capable of producing sen- 

 sation, or of occasioning contractions of the fibres, 

 are all chemical agents, possessing the capacity of 

 decomposition ; such as light, caloric, chemical salts, 

 colours, percussion, compression, and the like. 



There is, then, great reason to conjecture that 

 these causes act upon the nervous fluid in a chemi- 

 cal way, by altering its state or composition ; and 

 this is the more likely, because the force of this 

 action is diminished by continuance ; and hence it 

 appears necessary that the nervous fluid should be 



