386 ELEMENTARY ZOOLOGY 



hemispheres over the mid- and hind-brain. Man's brain 

 is many times larger than that of all other known mam- 

 mals of equal bulk of body, and three times as large as 

 that of the largest-brained ape. In man and the higher 

 mammals the surface of the forebrain is thrown into many 

 convolutions; among the lowest the surface is smooth. 

 Of the organs of special sense, those of touch consist of 

 free nerve-endings or minute tactile corpuscles in the skin. 

 The tactile sense is especially acute in certain regions, as 

 the lips and end of the snout in animals like hogs, the 

 fingers in man, and the under surface of the tail in certain 

 monkeys. All the other sense-organs are situated on the 

 head. The organs of taste are certain so-called taste- 

 buds located in the mucous membrane covering certain 

 papillae on the surface of the tongue. The organ of 

 smell, absent only in certain whales, consists of a ramifi- 

 cation of the olfactory nerves over a moist mucous mem- 

 brane in the nose. The ears of mammals are more highly 

 developed than those of other vertebrates both in respect 

 to the greater complexity of the inner part and the size 

 of the outer part. A large outer ear for collecting the 

 sound-waves is present in all but a few mammals. A 

 tympanic membrane separates it from the middle ear in 

 which is a chain of three tiny bones leading from the 

 tympanum to the inner ear, composed of the three semi- 

 circular canals and the spiral cochlea. The eyes (fig. 150) 

 have the structure characteristic of the vertebrate eye, con- 

 sisting of a movable eyeball composed of parts through 

 which the rays of light are admitted, regulated, and con- 

 centrated upon the sensitive expansion, retina, of the optic 

 nerve lining the posterior part of the ball. The eye is pro- 

 tected by two movable lids. In almost all mammals below 

 the Primates there is a third lid, the nictitating membrane. 

 In some burrowing rodents and others the eye is quite 

 vestigial and even concealed beneath the skin 



