Senses and the Central Nervous System 



At the end of Chapter 6 we saw that a thorough understanding 



/_\ of Cetacean behaviour must be based on the study of the sensory 

 _Z- jX^organs. We have aheady discussed their most important sense — 

 hearing — at length so that we can now devote our attention to the other 

 senses and particularly to seeing. To explain what changes the Cetacean 

 eye has undergone in order to adapt to life in the water we shall first take 

 a brief look at the eye of man as a representative terrestrial mammal. 



Our own eye lies in a bony case, the orbital cavity. Eyelids, eyelashes 

 and eyebrows protect the delicate tissue of the eyeball against the harmful 

 influences of dust and sweat, while the lachrymal glands continually wash 

 and lubricate the eyeball with a thin film of tears. The tears, after bathing 

 the surface of the eye, are drained from its inner corner into the nose by 

 the lachrymal duct. 



The wall of the eye is composed of three coats (Fig. 1 16), of which the 

 outer sclera is fibrous and preserves the form of the eyeball. From it the 

 seven eye muscles which help to rotate and move the eyeball in its socket 

 run to the orbital wall. The outer coat is transparent in front where it 

 forms the cornea, i.e. the part of the eye which we can see. Below the 

 sclera is found the middle layer or choroid which is dark and richly 

 vascular and contains the main arteries, veins and lymphatic vessels of 

 the eyeball. It completely surrounds the globe except for a small circular 

 opening in front - the pupil. The circular band immediately surrounding 

 the pupil is the mottled iris. The pupil can be dilated or contracted to 

 admit the precise amount of light needed. 



The inner layer or retina contains the receptors of sight, and is essen- 

 tially nervous in structure and function. Light stimuli are transmitted to 

 the brain by way of the optic nerve, whose terminal nerve fibres are dis- 

 tributed over the entire retina. Immediately behind the iris is found the 

 lens, w4iich is suspended in this position by the delicate suspensory 

 ligament which blends with the transparent lens capsule and is attached 



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