FUNCTIONS OF QUADRUPEDS. 19 



iect the sounds, and transmit them to the fine membrane 

 called the tympanum, or drum, which is expanded upon a 

 circle of bones, over a polished reverberating cavity. This 

 part is affected by the vibrations of the external air, much 

 in the same manner as a drum is by the motion of the sticks 

 upon it. It is also furnished with braces, which strain or 

 relax it, as necessity requires, and thus accommodate its 

 tension either to loud or languid sounds. 



Nearly all quadrupeds, except the Seals and Manati, 

 have an external ear, which is furnished with muscles, 

 and consequently is more or less movable. The size of the 

 external ear is proportioned to the size of the head, and the 

 necessities of the animals. Those that are most defenceless 

 have always a very quick sense of hearing, In some of the 

 Bats, the external cartilage or valve is proportionally longer 

 than in other animals, and contains a second valve, in front 

 of the orifice, so as to give the animals the appearance of 

 having double ears. 



The organ of smell lies in the pituitary membrane, which 

 lines all the internal cavity of the nostrils. This is provided 

 with numerous vessels and nerves, and is continually moist- 

 ened by a mucous humour. As the nostrils communicate 

 behind with the throat, the air, in passing through them to 

 the trachea, carries along with it the volatile and odoriferous 

 particles which excite the actions of this organ. 



The sense of taste lies chiefly on the upper surface and 

 edges of the tongue, which is a 'fleshy body, supplied with 

 many nerves. These terminate on the upper surface in 

 numerous papilla?, which are properly the organs of taste. 

 Betwixt the nerves there are certain minute vessels that 

 supply the saliva : this keeps the 'tongue and mouth always 

 moist, and is of indispensable use in masticating the food. 



The sense of touch, although it is found over the 

 whole superficies of the animal body, should be considered 



C 2 as 



