32 CEREBRAL FORCES. [i. 



stimulate a nerve to the performance of its function, although 

 in reality they, act strongly physically ; as, for example, a sound 

 which shakes every bone in the head, excites no animal actions 

 in the optic nerves. There is also the mode in which a nerve 

 receives an external impression j for the working of an animal 

 force in the medulla of the nerve is one thing, the propagation 

 to the brain of the impression received by the nerve, another. 

 Then there is nothing in the medulla whereby this transmission 

 can be explained according to mechanical or physical laws. The 

 medulla is neither hard nor elastic, but a soft body, which 

 according to the laws of physics must prevent or arrest the 

 communication of motion. Besides, this transmission takes 

 place so rapidly, and so soon after the external impression is 

 received, that the mind can perceive no space of time to occur 

 between the stimulation of the nerve, and the animal action 

 excited in a part of the body far distant from the point where 

 the impression was made. Nor can this transmission be effected 

 like a motion in fluids, for the medulla is not fluid matter, nor 

 so filled with fluid as to have the mobility of fluids, but is a soft 

 material which retards motion. Lastly, the properties of ethereal 

 fluids are not observable in the medulla, nor even in the vital 

 spirits, as, for example, such as ether, the electric fluid, &c., 

 which transmit motion in an unknown physical way. (Haller^s 

 ' Physiology,' § 379.) Since both the external impression and 

 its transmission along the nerves are operations of the vis nervosa 

 (6), and the aggregate of the animal forces in animal bodies is 

 termed their Senselikeness \_Sinnlichkeit] , it follows that the 

 mode in which the medulla of the nerve receives impressions 

 generally, and external impressions particularly (31), as well as 

 the mode in which it transmits them, together indeed with the 

 impression itself, (it being an animal force,) belong to the Sense- 

 likeness [Sinnlichkeit] of animal bodies, and cannot be deduced 

 from or explained by the mechanical and physical laws of motion. 

 33. There is no difference between the nerves of motion 

 and sensation in respect to the method of receiving and trans- 

 mitting external impressions (14, 31). — See Haller's 'Physiol./ 

 § 384. But as in the present section we have to consider the 

 animal forces abstractedly, and without reference to their motive 

 force on the mechanical machines (16), that which has been 

 stated must be understood to apply to the motor nerves only, 



