PHYSIOLOGY. 27 



10 sensation. 2. The brain (XXIX. 1.) seems to be a part 

 fitted for, and susceptible of those motions with which sensation, 

 and the whole consequent operations of thought are connected, 

 and thereby is fitted to form a communication between the mo- 

 tions excited in the sentient, and those, in consequence, arising 

 in the moving extremities of the nerves, which are often remote 

 and distant from each other. 3. The moving extremities 

 {XXIX. 4.) are so framed as to be capable of contraction, and 

 of having this contraction excited by motion propagated from 

 the brain, and communicated to the contractile fibre. -" It has 

 been disputed what connexion the moving extremities of mus- 

 cular fibres have with the brain, and the causes of their con- 

 tractility in consequence of this. It is true, that a muscular 

 part, being cut away from the body and from all mechanical 

 communication with the brain, will still retain its peculiar con- 

 tractility for some time. This has given occasion to many to 

 believe that the power of contractility has no dependence upon 

 other parts of the nervous system, but is something inherent in 

 the muscular fibre itself, which has been called a vis insita. 

 But without here making any distinctions between the vis in- 

 sita and the vis nervea, it is only to the last that I here refer."" 

 4. The nerves, more strictly so called (XXIX. 2.), are to 

 be considered as a collection of medullary fibres, each enveloped 

 in its proper membrane, and thereby so separated from every 

 other as hardly to admit of any communication of motion from 

 any one to the others, and to admit only of motion along the 

 continuous medullary substance of the same fibre, from its origin 

 to the extremities, or contrariwise. 



XXXV. From this view of the parts of the nervous system, 

 of their several functions and communication with each other, it 

 appears, that the beginning of motion in the animal economy is 

 generally connected with sensation ; and that the ultimate effects 

 of such motion are chiefly actions depending immediately upon 

 the contraction of moving fibres, between which and the sentient 

 extremities, the communication is by means of the brain. Where- 

 fore, in studying the nervous system, we judge it proper to con- 

 sider, 1. Sensation, and, with that, the general function of the 

 sentient extremities. 2. The action of the moving fibres. 3. 

 The function of the brain, In considering these three heads, 



