116 SENSATIONS. 



nected with motion. He esteems them to be, moreover, the centre of 

 consensual or instinctive movements, or of automatic movements involv- 

 ing sensation; a topic which will receive attention elsewhere. 



Having arrived at a knowledge that in man and the upper class of 

 animals, perception is effected in a part of the encephalon, our acquaint- 

 ance with this mysterious process ends. We know not, and we probably 

 never shall know, the action of the brain in accomplishing it. It is cer- 

 tainly not allied to any physical phenomenon; and if we are ever justi- 

 fied in referring functions to the class of organic and vital, it may be 

 those, that belong to the elevated phenomena, which have to be con- 

 sidered under the head of animal functions. We know them only by 

 their results : yet we are little better acquainted with many topics of 

 physical inquiry; with the nature of the electric fluid for example. 



The organs, then, that form the media of communication between 

 the parts impressed and the brain, are the nerves and spinal marrow. 

 M. Broussais, 1 indeed, affirmed, that every stimulation capable of causing 

 perception in the brain, runs through the whole of the nervous system 

 of relation; and is repeated in the mucous membranes, whence it is 

 again returned to the centre of perception, which judges of it according 

 to the view of the viscus to which the mucous membrane belongs; and 

 adapts its action as it perceives pleasure or pain. 



We are totally unacquainted with the material character of the fluid, 

 which passes with the rapidity of lightning along nervous cords; and 

 it is as' impossible to describe its mode of transmission, as it is to depict 

 that of the electric fluid along a conducting wire. As in the last case, 

 we are aware of such transmission only by the result. Still, hypotheses, 

 as on every obscure matter of inquiry, have not been wanting. 2 Of 

 these, three are chiefly deserving of notice. The first, of greatest anti- 

 quity, is, that the brain secretes a subtile fluid, which circulates through 

 the nerves, called animal spirits, and which is the medium of commu- 

 nication between the different parts of the nervous system ; the second 

 regards the nerves as cords, and the transmission as effected by means 

 of the vibrations or oscillations of these cords; whilst the third ascribes 

 it to the operation of electricity. 



1. The hypothesis of animal spirits has prevailed most extensively. 

 It was the doctrine of Hippocrates, Galen, the Arabians, and of most 

 of the physicians of the last centuries. Des Cartes 3 adopted it energe- 

 tically ; and was the cause of its more extensive diffusion. The great 

 grounds assigned for the belief were; first, that as the brain receives 

 so much more blood than is necessary for its own nutrition, it must be 

 an organ of secretion; secondly, that the nerves seem to be a conti- 

 nuation of the tubular matter of the brain; and it has already been 

 remarked, that Malpighi considered the cortical neurine to be follicular, 

 and the medullary to consist of secretory tubes. It was not unnatural, 

 therefore, to regard the nerves as vessels for the transmission of these 

 spirits. As, however, the animal spirits had never been met with in a 



1 Traite de Physiologic, &c., Paris, 1822; or translation by Drs. Bell and La Roche, 3d 

 Amer. edit., p 63, Philad., 1832. 



a Fletcher's Rudiments of Physiology, P. ii. b. p. 68, Edinb., 1836. 

 3 Tractatus de Homine, p. 17, Lugd. Bat., 1604. 



