346 SPECIAL PHYSIOLOGY. 



objects and phenomena; for without such internal and external corre- 

 spondences, no real knowledge would be possible, and certainly no com- 

 munity of knowledge, ideas, or thought, between different individuals 

 or generations, could exist. 



Concerning the completeness of the human senses, in regard to ex- 

 ternal nature, no certainty can ever be attained ; for, if there be agen- 

 cies in nature, other than those which now produce sensations within 

 us, it is impossible to prove their existence, unless our organization 

 were so changed as to enable us to perceive them. It is, however, as 

 unphilosophical to suggest a limit to the number of modes of action of 

 the common force of nature, as it is to assume the existence of such 

 modes as we cannot possibly establish by proof; for we cannot deny 

 the existence of other modes of action of the force of nature, which 

 may be unperceived only because they are imperceptible to us. But 

 the mind naturally inclines to a belief in a certain completenes in the 

 series of our senses, and in the harmony established between them and 

 the modes of action of the common energy of external nature. More- 

 over, there is reason to believe that the nervous energy manifested 

 in nervous reaction, is itself correlated with that common force. 



With regard to the possibility of the possession of additional external senses 

 by animals (for no one believes them to possess additional internal senses), it 

 may be stated that this is a question beyond our means of investigation. How- 

 ever, as a conclusion, based on general considerations, Man, being undeniably 

 the most highly organized creature on the earth, it is highly improbable that 

 the nervous systems of any of the animals lower than himself, are endowed 

 with special sensorial powers, enabling them to recognize modes of action of 

 the common force of nature, which pass undetected by him, because they are 

 inoperative upon his sensorial nervous substance. Man probably is endowed 

 with every kind of sensation possessed by animals, though each special sense 

 may, in certain animals, be more highly developed. In the warm-blooded 

 Yertebrata, the same senses undoubtedly exist as in Man, though modified in 

 acuteness in different instances. In the cold-blooded Yertebrata, especially in 

 Amphibia and Fishes, the low temperature of the body would seem incom- 

 patible with so high a grade of sensibility as exists in the warm-blooded spe- 

 cies, unless, indeed, their nervous substance be differently endowed. The in- 

 fluence of a low temperature is probably more felt in regard to common sen- 

 sibility, touch, taste, and smell, than in hearing and sight, which, so far as 

 the structure of their recipient organs, the ear and the eye, will permit, may 

 be very acute in these animals. Still lower in the animal series, as in the 

 aquatic Mollusca, the similarity of temperature between the body and the ex- 

 ternal medium, and the simplification of the sensory apparatus, are doubtless 

 associated with a further diminution of activity in the function of sensation. 

 In the Molluscoida, special sensation is probably almost entirely replaced by 

 common sensibility. In the Annulose animals, those in which the tempera- 

 ture is comparatively high, viz., the Insects, probably enjoy, not only greater 

 special sensibility, particularly as regards sight, but also a more acute common 

 sensibility than the colder aquatic Annulosa, such as the Crustacea and Annel- 

 ida. In these animals, movements excited by external irritation by no means 

 imply the acuteness of sensibility which is generally supposed. In the Annu- 

 loid animals, and also in the Crelenterata, special sensibility can only be feebly 

 manifested by those few species which possess simple ocular spots, and the so- 

 called auditory sacs ; whilst their common sensibility must be extremely feeble. 

 In the Protozoa, sensibility to light, or the warmth which accompanies it, is 

 noticeable in the Infusoria ; but in regard to any other sensation there is no 

 distinct evidence of its presence in these the lowest animals. 



In conclusion, it may be added, that in no animal are actions or movements 



