336 SPECIAL PHYSIOLOGY. 



consciousness of these central changes, produced within us by means 

 of such impressions, constitutes sensation. Changes, originating in 

 the sensorial centres, may also induce internal sensations. Hence, 

 in the act of sensation, whether from external or internal causes, we 

 feel the condition of our nerves and nervous centres, and not the ob- 

 jects or stimuli which excite them. 



The ultimate evidence of the existence of sensation, in each indi- 

 vidual, is intrinsic and personal. Conscious sensation is a fact, in 

 the constitution of our corporeal and mental nature, which is abso- 

 lutely incapable of explanation. The evidence of sensation in other 

 persons is a matter of inference from likeness of organization and 

 similarity of behavior under like impressions; or it rests on testimony 

 as to what they feel. The sensation of others, and their identity with, 

 or similitude to, our own, can neither be known nor demonstrated ; 

 they can only be inferred or assented to. As regards animals, the 

 existence in them, of sensations, whether common or special, is merely 

 inferable from the facts of their organization and their conduct. 

 Many of our ordinary sensations are composed of sensations associated 

 with certain ideas or judgments. 



The conditions of sensation are anatomical, physiological, and causal. 

 First, there must exist a sensitive surface or organ, to which the pe- 

 ripheral extremities of the sensory nerve-fibres may be distributed ; 

 secondly, such sensory nerve-fibres; and, thirdly, a sensorial gray 

 nervous centre; moreover, these several parts must be in anatomical 

 connection with each other. Such parts, for example, are the eye, 

 the optic nerve, and certain portions of the cerebrum. Even where, 

 as in the exceedingly simple eyes of some of the lower animals, the 

 ocular spots are placed close upon the nervous ganglia, there are 

 doubtless extremely short connecting nerve-fibres between them. 

 Secondly, the physiological conditions of sensation are, the healthy 

 state of the several specially endowed anatomical organs, a due supply 

 of arterial blood to those parts, and an active state of their nutrition ; 

 for, if they be either fatigued or inflamed, over-excited, exhausted, or 

 atrophied, sensation is modified, or even suspended. Abnormal con- 

 ditions of that part of the sensorium which is concerned in any par- 

 ticular sensation may cause increased or diminished sensibility to ex- 

 ternal or internal impressions. Lastly, the several parts just enumer- 

 ated as constituting a complete sensorial apparatus, .must be excited 

 by some causal agent, known as a stimulus, which, in the ordinary acts 

 of sensation, induces a change in the nerves, and through these in the 

 sensorial nervous centres, but which, in the case of internal sensations, 

 may act either on the nerves or directly upon the nervous centres. 



Sensory stimuli are said to be either external or internal, according 

 as they proceed from without or from within the body. The former 

 have also been named objective the latter subjective ; but, as elsewhere 

 already mentioned, even internal stimuli are equally objective in refer- 

 ence to the mind, which is, in the metaphysical sense, the real subjec- 

 tive element of our nature : to this, the body, and even the nervous 

 system, and the sensorium itself, are truly objective, and stimuli pro- 

 ceeding from them are therefore likewise objective, though internal or 



