40 CEREBRAL FORCES. [i. 



use in the animal economy, by means of other animal opera- 

 tions (42) ; and as, in fact, experience teaches us, that many parts 

 well supplied with nerves have little sensation, feeling but rarely, 

 and only a few external impressions, and those of a special kind, 

 as the heart, stomach, &c., it is very probable, that in animals 

 in a state of health many external sensations are prevented by 

 similar natural obstacles, and that this is no abnormal condition 

 of many nerves. That this important matter may be placed 

 in its proper light, we will endeavour in every possible way to 

 demonstrate from observations, how external sensations are 

 prevented naturally. 



i. Nature protects many nerves from contact by coverings, 

 by envelopes of cutis, or mucus, or so distributes them, that they 

 are only exposed to slight or gentle contact, or to certain im- 

 pressions expressly adapted to them, and little, if at all, to any 

 other. By this means, also, external sensations are so moderated 

 as not to be painful (46, i) . 



ii. There are many nerves, so situated and distributed, that 

 they are only exposed to certain agencies, the optic for example ; 

 which, in general, are only susceptible of external impressions 

 from the rays of light ; while the nerves of the skin receive no 

 impressions from the rays of light (40). In the same way, the 

 undulations of the atmosphere, which duly act as impressions on 

 the auditory nerves, cause no external impression on the delicate 

 and sensitive nerves of vision. The odorous particles which are 

 so perceptible by the olfactory nerves, have no effect on the 

 tactile, gustatory, auditory, or visual nerves. Sometimes certain 

 nerves are endowed for a period only, with the capability of 

 receiving external impressions from certain irritations and 

 influences, which they afterwards lose, as, for example, in the 

 sensational instincts (265). 



iii. Further, certain external impressions act so feebly on 

 nerves otherwise sensitive, that they do not go onwards to the 

 brain, but are weakened or lost in their course thereto. That 

 this feeble influence on the nerve has certainly excited an 

 external impression, is made clear by other animal actions, as, 

 for example, by certain animal movements which the impression 

 excites ; and the cause of its not being felt must be in its not 

 having reached the brain. Flatus in the stomach often excites 

 a tension of the nerves, which is so feeble, that we do not feel 



