THE SENSES 537 



but their position in the centre of the pupil would not appear 

 theoretically to be the most suitable position, and they must 

 have some other function. The horse, as far as we know, is the 

 only animal possessing them. 



Ligamentum Pectinatum. — Around the attached margin of the 

 iris — viz., at the corneo-scleral border — a peculiar spongy tissue 

 exists, which gives the iris at this part a distinctly elevated rim ; 

 this is known as the ligamentum pectinatum (Fig. 159). Roughly 

 speaking, it is a rim of spongy iris traversed by canals, crevices, 

 and spaces, which lead into the lymphatic system of the eye. The 

 function of this tissue is to carry off the aqueous humour as 

 rapidly as it is worn out and replaced, by which means the normal 

 tension of the anterior chamber is maintained. 



Intra-ocular Pressure has been estimated at 25 mm. of mercury. 

 This condition of tension is a necessary factor in keeping the 

 various structures of the eye in position, while an increase above 

 the normal leads to rapid destruction of vision. 



The Choroid Coat contains the vessels which nourish the 

 retina ; it possesses innumerable nerves, numerous lymphatics^ 

 and, further, it is an elastic coat. At its anterior part, behind the 

 iris, it forms the peculiar folded structure known as the Ciliary pro- 

 cesses, and in front of this it furnishes the tissue which is called 

 the Iris ; the iris and ciliary processes are therefore part of the 

 choroid coat. With the exception of one area, the whole of the 

 interior of the choroid is covered with pigment, and the same 

 extends on to the processes and iris. The area which is an 

 exception lies on the posterior wall of the eyeball above the 

 optic nerve ; it is of a brilliant colour, being a mixture of green, 

 yellow, and blue, and is known as the tapetum lucidum (Plate L, 

 Fig. 4*). This is found in both herbivora and carnivora ; in 

 the former it is due to the interference of light causing iridescence, 

 produced by the arrangement of the connective-tissue fibres of 

 the choroid, and not to the presence of any pigment ; in carnivora 

 it is due to minute crystals in the cells of the part, the crystals 

 causing the interference. The use of the tapetum is generally 

 supposed to enable animals to see in the dark ; this, of course, is 

 impossible, but it is probable that its presence enables an animal 

 to see better in a dim light. 



The Ciliary Zone is a peculiar and important part of the eye, 

 formed on the one hand by the junction of the cornea and 

 sclerotic, and on the other by the iris and ciliary processes. 

 Between these lies a muscle known as the ciliary, which is 

 firmly attached to the corneo-scleral margin, and runs back- 

 wards into the choroid, where it is attached. In man the 



* Berlin, Zeitschrift fur Vergleichende Augenheikunde, Heft 2, 1882. 



