

ORGANS OF THE SENSES. 247 



only upon the distal terminations of the symmetrical sensi- 

 tive filaments of the great nervous axis of animal life. In 

 the vertebrated animals these optical, acoustic, and other in- 

 struments destined to modify the external impressions so as 

 to produce more distinct perceptions, are mostly placed at 

 the ends of the sensitive nerves which issue from the inter- 

 vertebral foramina of the cranium, and are supplied by other 

 nervous branches indispensible to their function. The cir- 

 cumstances which necessitate the existence of such organs 

 in animals, also require them to be more numerous and va- 

 ried in higher than in lower tribes, and to be most perfect 

 and delicate where the locomotive powers, and consequently 

 the dangers are greatest. Hence they are more developed 

 in the active insects and other entomoid articulata, than in 

 the slow and torpid mollusca, and are most numerous and 

 perfect in the vertebrated animals where they have to watch 

 over the most complicated and delicate forms of organisation. 

 The columns of nerves appropriated to sensation are greater 

 than those of motion throughout the animal kingdom, and 

 they are spread more extensively through every part of the 

 body, so that almost every point is sensitive to impressions 

 of the density or resistance of outward bodies, to the feel- 

 ings of heat and cold, and to that of pain when they are in- 

 jured. This general sensibility, which watches over the 

 well-being of every part of the body, is most acute in the 

 skin, the common covering of all the organs, and the sensa- 

 tions belonging to all the senses are but modifications of 

 this, as their organs also are mostly developments from the 

 cutaneous covering of the body. As the apparatus for diges- 

 tion are the most important to the maintenance of life, and, 

 next to the cutaneous covering, the most universal in the 

 animal kingdom, the general sense of touch is probably first 

 modified or speciallized at their entrance, to constitute that 

 of taste which most immediately relates to this function; 

 and so are successively developed the various other senses 

 of animals, as those of smelling, hearing and vision, of which 

 we are conscious, and which make known the physical or the 

 chemical properties of external objects at greater and greater 

 distances from the sentient body. The sentient nerves are 

 thus very differently modified at their peripheral expanded 

 terminations to adapt them for receiving impressions from 



