258 



ANATOMY OF VERTEBRATES. 



205 



attains the back part of the capsule, and ramifies richly thereon, 

 in the foetus. 



The aqueous humour lodged in the chamber between b and /, 

 fig. 193, has a refractive power very little higher than that of 

 water; 100 parts consisting of 98'10 of water, 1-15 of chloride 

 of sodium, and 0'75 of extractive matter soluble in water, with the 

 merest trace of albumen : it is secreted by the membrane lining 

 the chamber. 



B. Appendages of the Eye. The muscles moving the human 

 eyeball are the four straight and two oblique ones. In lower 



Quadrumana a few fibres 

 seem to be detached from 

 the inner part of the origin 

 of the recti to be inserted 

 into the sclerotic nearer the 

 entry of the optic nerve. 

 This is the remnant of 

 a stronger muscle, which 

 in other Mammals, with 

 few exceptions, surrounds 

 the optic nerve, expand- 

 ing, funnel-wise, as it ap- 

 proaches the back of the 

 eyeball : it is called the 

 ( choanoid muscle,' or sus- 

 pensor oculi, and is supplied 

 by a branch of the sixth 

 cerebral nerve. In Cetacea it is divided into four short mus- 

 cles, paralleling the longer recti, but of greater breadth and 

 almost continuous : they are inserted into the sclerotic behind 

 the transverse axis of the eye-ball. The narrower and longer 

 recti muscles expand to be inserted anterior to that axis. The 

 superior oblique arising, with them, above the foramen opticum, 

 has the course of its fibres changed, as usual, by a pulley at the 

 upper and fore part of the orbit, but in passing through the sub- 

 stance which serves as the trochlea, the muscle is only partially 

 tendinous- and little diminished in diameter. The inferior oblique 

 is long, and broad at its insertion. 



In the Rhinoceros the fasciculi of the .choanoid muscles have 

 coalesced into two masses: in most quadrupeds they form a 

 single f infundibular suspensor.' The cellular tissue is more 

 or less condensed between the insertions of the choanoid and 

 the fleshy parts of the recti muscles, and in Man between these 



Arrangement of fibres of lens, Mammal, ccxin. 



