174 NATURE AND MAN. 



the sensorium, that luminous impressions are communicated to 

 our consciousness ; and this change is essential to the continued 

 nutrition of the tissue ; for it is well known to the physiologist, 

 that if an opacity on the front of the eye completely prevent the 

 access of light to the interior, the retina and the optic nerve 

 gradually waste away, just as muscles do when long disused. 

 What the precise nature of this change may be, is yet beyond our 

 ken ; but of the immediate and direct relation of light to the 

 peculiar properties of animal bodies, a very remarkable proof has 

 been recently given by the researches of one of the best experi 

 mental physiologists of our time, M. Brown-Sequard ; for he has 

 found that the contraction of the fibres of the iris, which diminishes 

 the diameter of the pupil, is capable of being called forth, not only 

 by the stimulus of light upon the retina, which affects the iris 

 through the nervous circle of reflex action, but also by the impact 

 of Light upon the iris itself, which directly excites the contraction 

 of its muscular fibres, in the same way as electrical or mechanical 

 stimulation excites muscular contraction elsewhere. 



By these most important links of connection, we are conducted 

 to another division of the inquiry that which relates to the powers 

 of Life. There have not been wanting, at any period in the history 

 of physiology, men who have attempted to identify all the forces 

 actii.g in the living body with those operating in the inorganic 

 universe. Because muscular force, when brought to bear on the 

 bones, puts them in motion according to the laws of mechanics 

 and because the propulsive power of the heart drives the blood 

 through the vessels on strictly hydraulic rules, it has been imagined 

 that the movements of living bodies may be fully explained on 

 physical principles ; no account being taken of the most im 

 portant consideration of all, namely, the source of that power 

 which the living muscle possesses, but which the dead muscle is 

 utterly incapable of exerting. So, again, because the digestive 

 process, whereby food is reduced to a fit state for absorption, and 

 the formation of various products of the decomposition that is 

 continually taking place in the living body, may be imitated in the 

 laboratory, it has been supposed that the appropriation of the 

 nutriment to the production of living tissue, and the various meta 

 morphoses which this undergoes, are to be regarded as chemical 



