lo Nov., 1908.] Elements of Animal Physiology. 679 



cf which are kept lubricated by a secretion formed in a special gland 

 — the lachrymal or tear gland. Painful sensations arising from the coxnea 

 give rise reflexly to closure of the lids and copious secretion of tears. The 

 cornea, it may be observed, possesses no blood vessels. Covering the 

 sclerotic in front {i.e., the white of the eye) is a thin sheet allied to the 

 mucous membranes called the conjunctiva. This is reflected on to the 

 under surface of the lids and is richlv supplied with blood vessels which 

 are capable of wide alterations in diameter. The curvature of the cornea 

 is not regular ; it resembles more the bowl of a spoon than the surface of a 

 sphere. The direction of the different curvatures varies in different animals. 

 Within the dome of the cornea is a watery transparent fluid — the aqueous 

 HUMOUR. This fluid is under some pressure and so maintains the convexity 

 of the cornea and separates this from the next structure to be mentioned. 

 Floating in the hindmost region of the aqueous humour is a perforated 

 curtain or diaphragm called the iris. The iris in all animals (except 



Fig. 57. The Eye. — C, cornea ; A, aqueous humour ; I, iris ; L, crystalline lens ; 

 V, vitreous humour ; R, retina ; S, sclerotic ; 0, optic nerve. 



albinos) is deeply pigmented and is opaque to light. It contains muscular 

 fibres, disposed radially and circularly, which by their contraction can 

 widen or narxow the central aperture called the pupil. The iris is plenti- 

 fully supplied with blood vessels, and by two sets of nerves, a cranial auto- 

 matic supply, from the third cranial nerve, which constricts the pupil, and 

 fibres from the thoracic autonomic which dilate the pupil. The use of the 

 iris is twofold ; it can regulate the amount of light entering the eye and 

 thus shield the delicate receptors within from injury through excess of light, 

 and secondly, when the illumination is sufficient, it can help in forming a 

 clearer retinal image of the outside world by shutting off all rays of light 

 except a narrow central beam — an action which will be familiar at once to 

 all who have had anything to do with photography. The iris works re- 

 flexly, that is, automatically. When the retina is in darkness or is feebly 

 illuminated, the pupil is widely dilated ; if the light entering the eye in- 

 creases in intensity the pupil constricts correspondingly. The shape of 

 the constricted pupil varies with different animals, being circular in man 

 and the dog ; slightly oval in the rabbit ; having the form of a slit which is 

 vertical in the cat, but horizontal in most of the domestic animals. The 

 iris responds to drug treatment giving a dilated pupil with atropine, cocaine 

 and adrenalin and in the latter stages of chloroform ansesthesia, but a 

 narrow pupil w'ith eserine, and opium and in the earlier stages of chloro- 

 form anaesthesia. It also varies to some extent with the blood-pressure and 

 the state of the emotions, e.g., fright. The movements of the iris serve this 



