846 



PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



The intensity of the sense of smell depends, first, upon the size of 

 the olfactory surface, since we iind that in animals in which the sense 

 of smell is most acute the turbinated bones of the olfactory region are 

 most complicated ; secondly, on the concentration of the odorous sub- 

 stance in the air; and, thirdly, on the frequency with which the columns 

 of air containing the odorous particles are conducted to the olfactory 

 organs; hence sniffing tends to increase the intensity of odors. 



The development of the sense of smell is always more highly marked 

 in animals than in man and plays an important part in their organization. 

 Game-dogs, as is well known, will recognize the odor from game-birds 

 at several hundred yards, but even this falls below the acuteness of smell 

 possessed by various animals which are able to scent the presence of 

 man at a distance of a mile or more. 



B. THE SENSE OF SIGHT. 



Vision is the perception of the sensation caused by the impression 

 of a ray of light upon the retina, and in all animals depends upon the 

 special sensitiveness of the optic nerve-filaments to the vibration of 

 luminous rays. Animals may, nevertheless, be sensible of light without 

 special organs of vision, and may even be capable of giving evidence of 

 the impression of such light ; thus, the hydra, although it has no distinct 



organs of vision, will move around 

 from side to side of the vessel in 

 which it is placed until it has 

 reached that on which the sun is 

 shining and will turn itself toward 

 the seat of light. 



In its simplest form the visual 

 apparatus is represented b} r a col- 

 lection of pigment-cells in the 

 outer coverings of the body which 

 are in connection with the ter- 

 mination of afferent nerves. The 

 pigment absorbs the rays of light, 

 and in that function some process, 

 probabl}- of a chemical nature, 

 is excited and the sensitive nerves are stimulated. In the medusre, 

 as the jelly-fish, and at the ends of the rays of the star-fish and other 

 echinoderms similar collections of pigment are found, but as the 

 lens is wanting no distinct image can be formed, and, consequently, 

 in such cases the distinction between light and darkness is all that 

 is possible. In some of the cephalopod mollusks two simple eyes, 

 consisting of a globular lens with transparent media analogous* to the 



Fig. 365. — Head and Compound Eyes of 



the Bee. (Carpenter.) 



The ocelli are in -litii on the one side (A I and displaced on the 



other (B) ; A A A, stennnata; B B, nntennze. 



