CHAP. IX.] NERVOUS SYSTEM AND OBGANS OF SENSE. 313 



action of these nerves is paralysed (as, c.r/., by division) the arteries 

 they supply dilate, and in this way increased sensibility and secreting 

 power is promoted by the greater supply of blood thus induced. 

 Some physiologists, however, explain this dilatation as a positive 

 instead of a negative action. 



The nervous sytem is, as we have seen, bilateral, but between the 

 two halves of its central part there are, as we have also seen, 

 numerous transverse commissural structures and decussating fibres. 

 The correspondence of function with structure in this respect is 

 shown by those cases, already given, in which injury or stimulation 

 of one side of the central nervous system produces effects on the 

 opposite side of the body. That the cerebral hemispheres are each 

 capable of carrying on all the activities requisite for cognition is 

 proved by the fact that the destruction of one does not necessarily 

 even impair cognitive activity. It may seem strange that with the 

 presence of a double organ of such a special nature, processes of 

 cognition and feeling should be, as they are, single ; but, in fact, 

 there is no greater wonder in cognizing singly by two similar hemi- 

 spheres than in seeing singly through two similar eyes. 



§ 37. A phenomenon that may not unfitly be here noticed, is 

 SLEEP, since during it the central nervous system, or part of it, 

 undergoes a temporary suspension of its activity. It is the cerebrum 

 and the sensory centres which thus periodically intermit their action. 

 Certain parts, however, of the nervous centres never sleep, as is the 

 case with those which govern the imceasing respiratory actions, as 

 also those parts of it which minister to all the functions of vegetative 

 life. Yet, although thus continued, the frequency of the respiratory 

 and 'circulating movements, diminishes, and the temperature of the 

 body is consequently lowered. The quantity of the blood in the brain 

 is by some persons supposed to be diminished during sleep. Before 

 the daily period of repose is reached, the activity of the nervous 

 system has generally already been so taxed that it no longer 

 responds with thorough readiness to stimuli. With the assumption 

 of an attitude of rest, and -svith the closing of the eyes, this 

 torpidity increases, till it ends in more or less complete repose. 

 Sleep gradually counteracts the efl'ects of previous activity, and in 

 consequence the readiness to respond to stimuli becomes progres- 

 sively greater as sleep continues, till the occurrence of some very 

 slight exciting cause sufiices to awake the dormant animal. Apart, 

 however, from any exhaustion from fatigue, the cat generally falls 

 asleep \\dth remarkable readiness at any hour (when not disturbed 

 by hunger or passion), as soon as it has become quiescent. 



§ 38. Before concluding this account of the functions of the 

 nervous system by saying what remains to be said of the several 

 special senses, certain remarks may be made with respect to sensa- 

 tion IN general. 



Sensations are of two classes, external and internal. The former 

 are produced by the agency of external bodies, the latter by the 

 body itself. They may both exist simultaneously, as when one part 



