202 ANATOMY AND PHYSIOLOGY. 



I 



The Radiata, and almost all the Mollusca, appear to want this 

 sense, but it is possessed acutely by many insects, though the organ 

 used is not accurately known. In the sepia is found the simplest 

 organ of hearing. It is merely a sac- filled with fluid, with the nerve 

 expanded in it, and having a hard body attached to its extremity. 

 Fishes have this organ a little more complicated, but in neither these 

 nor the sepia is there any external opening. They hear as we do 

 when a hard body is held between the teeth, the conducting power of 

 water for sound being much greater than that of air. When the 

 Abbe Nollet sank his head under water and struck two stones 

 together, the shock to the ear was almost insupportable. This 

 organ becomes progressively more complicated in reptiles, birds, 

 and the mammalia. Among the last we first find external cartilages, 

 which, as well as the internal tube, are directed forwards in those 

 which pursue their prey, and backwards in timid animals, such as 

 the hare, rabbit, &c. 



VISION. 



The next and last sense we have to treat of is vision. All the 

 affections of this sense are derived from the action of light. We 

 think we see the bodies themselves that are scattered round us, but 

 this is a mistake, for they themselves have no color. The color, or, 

 more properly speaking, the power to produce the sensation we call 

 color, resides entirely in the rays of light that are thrown off or 

 reflected from these bodies to our eyes. In spite of our convictions, 

 however, we cannot help conceiving of our sensations as abiding 

 qualities in these different objects. 



If a ray of light be admitted through a small opening into a dark 

 chamber, it appears white, but by causing it to pass through a three- 

 sided piece of glass called a prism, it is seen to be composed of 

 different-colored rays. These, according to Dr. Wollaston, are 

 red, yellowish green, blue, and violet. In this way a ray of light 

 is decomposed ; when these colors are all uniformly blended, as 

 when a card on which they are separately painted is rapidly whirled 

 round, the resulting color is again white. Now, it is from the 

 power bodies possess of throwing off or of absorbing special rays 

 out of the number, that they appear to us differently colored. If a 

 body appears blue, the blue rays alone have been reflected ; and so 

 on w T ith red, green, and other colors. We do not notice any inter- 

 val between looking at an object and the impression on our eye (as 

 we can do with distant objects in the case of sound), from the 



