EXTERNAL SENSATIONS. 119 



of conveying electricity; and, moreover, when wires are inserted 

 into an exposed nerve, and their opposite extremities are attached to 

 the galvanometer, no movement of the needle has been observed by 

 Person, Muller, Matteucci, and by Todd and Bowman. 1 Dr. Bostock, 2 

 too, has remarked, that before the electrical hypothesis can be con- 

 sidered proved, two points must be demonstrated; first, that every 

 function of the nervous system may be performed by the substitution 

 of electricity for the action of nerves ; and secondly, that all nerves 

 admit of this substitution. This is true, as concerns the belief in the 

 identity of the nervous- and electrical fluids ; but we have, even now, 

 evidence sufficient to show their similarity ; and that we are justified 

 in considering the nervous fluid to be electroid or galvanoid in its na- 

 ture, emanating from the brain by some action unknown to us, and 

 transmitted to the different parts of the system to supply the expendi- 

 ture, which must be constantly taking place. 



Reil, 3 Senac, 4 Prochaska, Scarpa, 5 and others are of opinion, that 

 the nervous agency is generated throughout the nervous system ; and 

 that every part derives sensation and motion from its own nerves. We 

 have satisfactorily shown, however, that a communication with the ner- 

 vous centres is absolutely necessary in all cases, and that we can imme- 

 diately cut off sensation in the portion of a nerve included between two 

 ligatures, and as instantly restore it by removing the upper ligature, 

 and renewing the communication with the brain. 



a. External Sensations. 



The external sensations are those perceptions which are occasioned 

 by the impressions of bodies external to the part impressed. They are 

 not confined to impressions made by objects external to us. The hand 

 applied to any part of the body; any two of its parts brought into con- 

 tact; the presence of its own secretions or excretions may equally excite 

 them. M. Adelon, 6 has divided them into two orders first, the senses, 

 properly so called, by the aid of which the mind acquires its notion of 

 external bodies, and of their different qualities ; and secondly, those sen- 

 sations which are still caused by the contact of some body ; and yet 

 afford no information to the mind. 



It is by the external senses, that we become acquainted with the 

 bodies that surround us. They are the instruments by which the brain 

 receives its knowledge of the universe ; but they are only instruments, 

 and cannot be considered as the sole regulators of the intellectual sphere 

 of the individual. This we shall see is dependent upon another and 

 still higher nervous organ, the brain. 



The external senses are generally considered to be five in number ; 

 for, although others have been proposed, they may perhaps be reduced 

 to some modification of these five, tact or touch, taste, smell, hearing, 



1 The Physiological Anatomy and Physiology of Man, p. 242. Lond., 1845. 



2 An Elementary System of Physiology, 3d edit., p. 148. Lond., 1836. 



3 De Structura Nervorum, Hal., 1796. 



4 Traite de la Structure du Coaur, &c., liv. iv. chap. 8. Paris, 1749. 

 s Tabulse Neurologicae. Ticin., 1794, 22. 



Physiologic de 1' Homme, torn. i. p. 259, 2de edit. Paris, 1'829. 



