CH. II.] EXTERNAL SENSATIONS. 48 



place the material idea belonging to it, although it has arrived 

 at the brain ; that there are such impediments is certain, and 

 sleep is an example. This is a periodic state of insensibility 

 natural to all animals endowed with sensation, and enables them 

 to collect new strength after the weariness resulting from ac- 

 tivity, and which arises, as some think, from the want or 

 weakness of the vital spirits ; light may shine into the eyes, 

 sound fall on the ears, and the nerves may be stimulated in a 

 thousand waj^s, and yet no external sensation be excited. 

 Consequently, either the external impression never reaches 

 the brain (and for this conclusion, there is not the slightest 

 foundation), or else no material ideas are formed therein, 

 or at least only imperfectly ; and this is probably the true 

 doctrine, since a compression of the brain, either by haemor- 

 rhage, or effusion, or depression of a portion of the cranium, 

 or even excessive distension of the blood-vessels, develops the 

 same insensible state, and induces true sleep. It appears as if 

 the brain were in a state of torpor in profound sleep, so that 

 the material ideas are prevented being developed by the external 

 impression on the nerves ; whilst the purely animal movements 

 excited by the impression on the latter only appear to ex- 

 perience no change, — (see § 182, 183.) 



50. vi. There is still another special cause which prevents 

 external sensations arising in the mind, in one or other of the 

 five methods described (47-49) ; and this is the frequent repetition 

 of an external sensation. By this, as observation teaches, 

 many external sensations become gradually weaker and weaker, 

 and at last cease altogether, although the impression on the 

 nerve still takes place. This diminution and destruction of 

 external sensations by frequent repetition, is termed the habit 

 of external sensations, and since it cannot be explained on 

 mechanical principles, it must be classed with the properties 

 peculiar to animal bodies (6). 



51. Habit weakens or destroys external sensations in the 

 five following ways : 



i. By their frequent repetition the susceptibility of the 

 nerve may be weakened or prevented (47, i). For example, a 

 thick cuticle is developed, and protects the terminating fibrils, 

 in consequence of certain oft-repeated impressions on the organs 

 of touch. 



