204 ANIMAL FORCES. [ii. 



patient, still it is probable that sometimes external impressions 

 on the brain itself excite nerve-actions, and it may really be 

 capable in some degree of the vis nervosa of external impres- 

 sions. Many facts appear to corroborate this. ^'^A certain 

 person had a piece of bone driven into his brain. For a long 

 time no results followed, but at last spasms and death took 

 place from a large ulcer of the brain.'^ (Haller.) Still nothing 

 certain can be stated on this point (624, iv). It is more 

 probable, however, that an internal impression on the brain, 

 not proceeding from conceptions, excites, as nerve-actions, the 

 same movements that it would have excited if it had proceeded 

 from a conception, since this continually takes place in the 

 spinal cord, which is analogous to the brain (12^). Indeed, in 

 the case just mentioned, when an external impression, though 

 not felt, excites animal movements in the body, they must have 

 been caused by the external impression reflected independently 

 of the animal-sentient force, and without the intervention of a 

 conception, and acting as a mere internal impression. But 

 without laying much stress on this doubtful case, there are 

 many other reasons for recognising the existence of a vis 

 nervosa in impressions on the brain not caused by conceptions. 

 When in a plethoric person the brain becomes congested, as in 

 stooping, and the small arteries in the optic nerves are dis- 

 tended, and stimulate the origin or trunk of the nerves, this 

 internal impression in the brain is transmitted downwards to 

 the termination of the nerve in the eye, and there causes 

 an external impression which is transmitted upwards (31), and 

 excites in the mind an imperfect external sensation (148). This 

 imperfect external sensation of various false appearances before 

 the eyes, is a manifest proof, that internal impressions which do 

 not depend on conceptions, are transmitted through the brain, 

 and have the same actions as those which proceed from con- 

 ceptions ; for just as v&ry vivid imaginations, or passions, cause 

 (as sentient actions) imperfect external sensations of various 

 appearances before the eyes (148), so in this case, the pressure 

 of blood in the optic nerves at their cerebral origin acts as a 

 nerve-action, and sparks and motions are seen before the eyes 

 as vividly as if they were real. Now since the optic nerve is 



• Fide foot-note to § 34.— Ed. 



