206 ANIMAL FORCES. [ii. 



nerves, excites visible animal actions in the brain, without 

 producing a material external sensation at the origin of its 

 nerves. At least such actions are not observed in syncope, or 

 profound sleep (49), when light streams into the open eye, or 

 a loud noise strikes on the ear without being felt. Yet in these 

 cases some animal actions in the nerve must result, because 

 the external impression made on it goes to the brain, which is 

 of itself an animal movement (32), and because the sensational 

 force of the nerves may be impaired at their cerebral origin, when 

 in cases of this kind, the external impressions are too violent, 

 although they are not felt. Thus, for example, deafness 

 gradually comes on in persons who sleep in a mill, and blindness 

 from the gleaming of the moon's rays into the eyes during 

 sleep, &c. Only, these are not perceptible nerve- actions, but 

 are probably only irregularities of the vital spirits, or imper- 

 ceptible changes in the medulla of the nerves, and consequently 

 no definite conclusion can be drawn as to the vis nervosa of 

 external impressions situate in the sensory nerves. 



377. A non-conceptional internal impression on purely sensory 

 nerves, displays the traces of its actions distinctly enough, as 

 in the case of the nerves of vision previously mentioned (373). 

 All the visual and auditory phenomena so often noticed on 

 stooping, or whirling round too rapidly, or binding the neck 

 too tightly, are, in fact none other than animal actions (namely, 

 imperfect external sensations) in purely sensory nerves, excited 

 by non-conceptional impressions, which are usually produced by 

 conceptions, and exciting similar actions (148). Analogous 

 phenomena are manifested in the nerves of taste, and smell, 

 and touch, so that persons of great sensibility, and especially in 

 certain morbid states, think they perceive tastes or odours which 

 are not present, and are not caused by any conception. 



378. It was previously observed (150), that true external 

 sensations may be regarded as being imperfect, when an ex- 

 ternal impression is really made on the sensory nerves, but by 

 something within the organ of sense; as, for example, when 

 in inflammation of the eyes, or in retention of air in the ears, 

 phenomena are seen or heard which are not in fact so real as 

 it is thought. These may be termed generally imperfect ex- 

 ternal sensations from an erroneous judgment ; those mentioned 

 in the preceding section may be termed imperfect external 



