CH. I.] ANIMAL MACHINES. 19 



such machines to motion, if they be appropriated to certain 

 movements, or at least without co-operating therein. The 

 first are termed, in relation to their function, motor nerves; 

 the other are sensitive nerves. They are, however, identical 

 in structure, and only differ in this local relation. Each nerve 

 may be either the one or the other, according to its distribu- 

 tion ; and each motor nerve is at the same time endowed with 

 the properties of the sensitive. The motor nerves have ganglia, 

 composed partly of their own fibres, and partly of other nervous 

 twigs and nerves which accompany them, whereby the direct 

 course of the fibrils and nerves is interrupted. The nerves of 

 the senses, which have no motive influence on the mechanical 

 machines of the organism, have no ganglia. 



15. i. All the phenomena of motion and sensation mani- 

 fested through the nerves, render probable the existence of a 

 remarkably subtle fluid essence, which is present invisibly in 

 the medulla of the brain and nerves, and is the means whereby 

 all the functions of both are performed. It is termed the 

 vital spirits or nervous fluid, but it is not known how and 

 when it contributes to the animal actions. It is not that fluid 

 matter which is seen in the medulla of the brain and nerves, 

 but a much more subtle spirit, imperceptible to the senses. 

 It is inferred from the phenomena which betray its existence, 

 that this nervous fluid is a remarkably mobile fluid, a spirituous 

 vapour, which can be neither aqueous, nor glutinous, nor 

 elastic, nor etherial, nor electrical. 



ii. Although animal machines are indeed proper to all 

 animals (6), still every species does not possess those which 

 have been just described, but only the most perfect, namely, 

 man and the animals immediately below him. But since our 

 object is not to lay down the principles of a physiology of the 

 proper animal nature of man only, but rather of animals in 

 general, these principles will be found applicable to an expla- 

 nation of the functions of animal machines in the various 

 classes of animals, a detailed description of which may be 

 found in Haller^s greater Physiology, and in his ^ Opera 

 Minora.^ ^ 



From these statements we may draw the conclusion, with 



* The author here gives a sketch of the comparative anatomy of the nervous 

 system, as known in his day ; this has been omitted for the sake of brevity. — Ed. 



