242 THE ANIMALS AND MAN 



preponderance of the forebrain or cerebral hemispheres 

 over the mid- and hind-brain. Man's brain is many times 

 larger than that of all other known mammals of equal bulk 

 of body, and nearly 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 cov- 

 ering certain papillae on the surface of the tongue. The 

 organ of smell, absent only in certain whales, consists of a 

 ramification of the olfactory nerves over a moist mucous 

 membrane 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 

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

 panic 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 have the structure char- 

 acteristic of the vertebrate eye, consisting of a movable eye- 

 ball composed of parts through which the rays of light are 

 admitted, regulated, and concentrated upon the sensitive 

 expansion, retina, of the optic nerve lining the posterior 

 part of the ball. The eye is protected 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 con- 

 cealed beneath the skin. 



