224 ANIMAL FORCES. [ii. 



primarily received the impression, or through its own efferent 

 fibrils (127), the impression is transmitted along the nerve 

 upwards to the brain ; but ere it reaches there, it is turned 

 from its course, and so reflected downwards, that it excites (as 

 an internal impression) the nerve of the other remote parts, or 

 the nerve-twigs or efferent nerve-fibrils of the part receiving 

 the impression ; and this internal impression, which is nothing 

 else than the reflected external impression, thus reaches the 

 mechanical machine which has to perform the nerve-action. 

 This is proved by undoubted experiments. If the toe of a frog 

 at rest be pricked, the external impression thus made goes to 

 the brain. From thence it is reflected upon the limbs, and 

 the animal rises up and springs forward. But if the head be 

 cut off, and the toe be again pricked, the same motions take 

 place (357). In this case, the external impression on the toe, 

 must pass upwards towards the brain, although it cannot reach 

 it, for if the nerve be divided in the thigh, so as to prevent 

 its transmission, the motion does not take place. Further, it 

 is obvious that it is reflected on the nerves of the limbs, as an 

 internal impression, and along their twigs to their muscles, 

 because no other part of the body, except this single toe 

 which was pricked, receives an external impression. Again, 

 supposing while one toe is pinched, that the nerve of the other 

 leg be divided ; in this case, the movements will be repeated in 

 all the parts except that whose nerve is cut through. This 

 explains what takes place in similar circumstances, when an 

 impression is made on the spinal cord, and spasms and con- 

 vulsive movements are excited in all parts below the point of 

 irritation, except those the nerves of which are cut through. 

 The reflected external impression passes as an internal im- 

 pression to, and only excites movements in those muscles to 

 which it can be transmitted from the point of reflexion. 



iii. Examples of this class of nerve-actions are to be met 

 with daily, which sometimes are mistaken for sentient actions 

 (which they often accompany), sometimes for special operations 

 of unfelt external impressions acting through the brain, some- 

 times for inexplicable sympathies. Numerous instances of this 

 kind may be found in Haller^s ' Physiology,' vol. iv, p. 529, 

 and B. 10, Absch. vii, ^ 30-31. 



416. As the brain secretes the vital spirits, and as in 



