xxvi RECEPTORS 671 



The brain is absolutely larger in whales than in any other animals 

 (up to 7,000 g), and the hemispheres are elaborately folded. The 

 cerebellum is very large. Little is known in detail about the sensory 

 equipment or powers of the animals. The eyes are small (vestigial in 

 Platanista) and in all whales much modified for aquatic life and diving. 

 The cornea is more flattened than in subaerial mammals, and the lens 

 rounded; in these respects the whales have returned to fish-like condi- 

 tions. The whale eye is enclosed in a thickened sclera and further has 

 special lid muscles. The tear glands and their duct are absent; instead, 

 the surface of the eye is protected by a special fatty secretion of the 

 Harderian glands. 



The ear provides the major receptor system. The auditory nerve, 

 lateral lemniscus, superior olive, inferior colliculus, and medial genicu- 

 late are all very large. Presumably much of the cortex serves the sense 

 of hearing. The apparatus concerned with reception of air-borne 

 vibrations is reduced. The external opening is very small and the 

 long meatus is often filled with secretion. The tympanum is thick and 

 ligamentous. It is to a normal ear drum as a closed umbrella is to an 

 open one. The tip of the tympanic 'ligament' is attached only to the 

 tip of manubrium mallei. The distal end of the processus gracilis of 

 the malleus is fused to the adjoining bone of the tympanic bulla. The 

 ossicles have articulations with one another as in terrestrial mammals 

 and the tip of the stapes is movable in the foramen ovale. 



The petro-tympanic bone is free from the skull, rests on a thick 

 fibrous pad and is otherwise almost completely enveloped in a system 

 of foam-filled air sinuses. The whole arrangement is believed to be 

 designed to isolate the essential organ of hearing from vibrations 

 extraneous to those reaching it by means of the meatus and auditory 

 ossicles, and so to provide the means for directional hearing. 



In the ears of terrestrial mammals the ossicles provide an arrange- 

 ment for converting the relatively large displacement at low amplitudes 

 of air-borne waves at the tympanum to waves with a sixtyfold greater 

 pressure amplitude at the fenestra ovalis. The physical properties of 

 water-borne vibrations, however, differ markedly from those that are 

 air-borne. The pressure amplitude for the same intensity and fre- 

 quency of water-borne and air-borne sound is in the ratio 61 : 1, and 

 the displacement amplitude 1 :6i. It can be shown that adjustments of 

 amplitude and pressure to values normally experienced in the cochlea 

 by terrestrial mammals are achieved in cetaceans by the modifications 

 of the middle ear mechanism (Fraser and Purves, 1959). 



Whales emit a variety of sounds but little is known of their method 



