SENSE ORGANS 465 



of sight, hearing, smell, taste, and touch, there are distinct kinds 

 of sensibility to heat, cold, and the movements of the body, 

 an indefinite general sensibility and a sense of damage to the 

 tissues which in man is associated with the feeling of pain. 

 Each of these senses has origin in impulses derived from a special 

 kind of nerve ending, but only in the case of sight, hearing, and 

 smell are these endings situated in a highly specialised organ. 

 We shall here consider only these organs. 



EYES 



The eyeball of a mammal is best studied in that of the sheep 

 or ox (Fig. 365) ; it is roughly spherical, but flattened on the 

 front side. It consists of the following parts : (i) The outer coat 

 or sense capsule corresponds to the auditory and nasal capsules, 

 but fits closely to the eye instead of forming a hollow capsule 

 fused to the skull. Over the greater part of the eye it consists of 

 dense connective tissue with some cartilage and is known as the 

 sclerotic, but on the front side it is transparent and known as 

 the cornea. (2) The skin over the cornea adheres to it as a delicate, 

 transparent covering, the conjunctiva, which is kept moist by 

 the secretion of Harderian glands below the eye and lacrimal 

 or tear glands above the outer corner of each eye. There are no 

 Harderian glands in man. The secretions of these glands drain 

 from the conjunctiva into the nose. (3) Inside the sense capsule 

 is the choroid coat, consisting of looser and highly vascular 

 connective tissue containing numerous dark pigment cells. In 

 front the choroid thickens as the ciliary body and then separates 

 from the sclerotic and passes inwards, as a partition called the 

 iris, across the hollow of the eyeball, which it thus divides into 

 anterior and posterior chambers. The former is smaller and filled 

 with a watery aqueous humour, the latter larger and filled with 

 a gelatinous vitreous humour. In the middle of the iris is an 

 opening, the pupil, and the iris contains muscular tissue by which 

 the size of the pupil can be altered. (4) Immediately behind the 

 iris lies a firm, transparent body, the lens, which serves to focus 

 upon the sensitive surface at the back of the eye the light which 

 enters through the pupil. (5) The sensitive surface is provided 

 by the retina, a dehcate membrane containing two primary 

 layers, an outer pigment layer of pigmented cells lining the 

 choroid, and an inner retina proper which has at the back of the 



