chap, xii.] SMELLING AND SCENTING. 443 



olfactory membrane, we may estimate the comparative 

 complexity of the turbinate bones by the acuteness of 

 smell of their possessor. Many mammals, both those 

 that hunt (Felidse), and those that are hunted (Cer- 

 vidre), have a much more acute sense, and more com- 

 plicated turbinate bones, than has man (Fig. 189). 



Like other specialised sensory organs, the olfactory 

 apparatus of Vertebrates is provided with character- 

 istic cells, which are to be found in the lamprey almost 

 as well marked as in man. (See " Elements of His- 

 tology," Fig. 166.) 



In the physiology of this sense it is necessary to 

 distinguish between smelling;, which is a more or 

 less passive act, and scenting 1 , which is an active 

 operation. Although we cannot suppose that the 

 latter power is well developed among Fishes, yet the 

 fact that the nasal valve is provided with muscles, 

 taken in connection with what we know as to the 

 habits of sharks, for example, justifies in believing that 

 some fishes, at any rate, are capable of scenting as well 

 as of smelling. In the Sauropsida a more forcible in- 

 spiration of air must be the chief aid, but in Mammals 

 the addition of external movable cartilages supplied 

 with muscles results in a power to enlarge or diminish 

 at will the size of the entrance to the nasal passages. 



The external cartilaginous "nose" once formed may 

 become adapted to duties altogether foreign to the 

 olfactory sense ; it may be prolonged into a snout 

 which, as in the pig, may be of real use as a digging 

 organ, or it may become, as in the elephant, greatly 

 elongated, and have the functions of a prehensile trunk, 

 or proboscis. 



The sense of sight is at first a generalised property, 

 many Protozoa showing themselves to be sensitive to 

 light. The most primitive condition of an eye or optic 

 organ is presented by patches of pigment which are 

 more sensitive to light than is protoplasm generally. 



