PREFACE. 9 



the animal motive forces, or by and through themselves in the 

 entire mechanism of the body. This is peculiarly important 

 with reference to the sensations, and the instincts and passions, 

 and of consequence in every department of medical science. 



In the Second Part, the nerve-forces are treated of so far 

 as they act independently of the mind. It is shown that, 

 besides the animal-sentient forces of the brain, there are only 

 two kinds of forces of the nerves which act in the body as 

 animal- motive forces, namely, the sense-like [sinnlich] im- 

 pressions, which are divided into internal and external (Part II, 

 Chap. I). These two kinds of impressions are considered in 

 the Second and Third Chapters respectively; and in the Fourth, 

 their relations to the mental forces are elucidated. Modern 

 physiologists, whose names Europe knows and honours, — Haller, 

 Zimmermann, Whytt, and Oeder, — have rendered much ser- 

 vice to this department of physiology by contributing materials 

 thereto. Haller has, indeed, begun to trace out the plan of 

 this new department, which certainly did not exist before him ; 

 but there he has stopped. I have ventured to extend this 

 outline, with the hope of inducing able men to complete it. 

 The most important progress that has been accomplished in 

 this matter, consists in the following : I have defined the two 

 kinds of impressions, and the entirely diff'erent laws by which 

 they move the body, without having recourse to the hypothesis 

 of vital spirits as a motive power; for these sense-like [sinnlich] 

 impressions can be considered simply as phenomena, and their 

 laws of action discovered without a knowledge of their nature. 

 I have derived primarily from the nerves, that motive force of 

 the external impression, which Haller assigned to the muscular 

 fibres under the designation of irritability, but denied it to be 

 a property of the nerves ; I have demonstrated the deflection 

 [declination] and reflection of the impressions in the nerves, 

 whereby many phenomena of the animal economy, hitherto 

 inexplicable, can be understood; and I have shown how the 

 vis nervosa is sufficient of itself to develope those movements 

 in bodies, which were formerly attributed to the influence of 

 the mind or soul, and vice versa. 



I have added the Third Part to describe the economy of the 

 animal forces in general, and trace, as it were, the course of 

 life in animal nature. The First Chapter contains a sketch 



