COMPARATIVE ANATOMY. 



tion of the owl, who looks straight for- 

 wards. They are placed laterally in all 

 reptiles. Their situation varies much in 

 fishes : they look upwards in the uranos- 

 copus : they are both on the same side of 

 the body in the pleuronectes ; hut in ge- 

 netal their direction is lateral. 



The form of the globe varies according 

 to the medium in which the organ is to 

 be exerted. In man and the mammalia, 

 it deviates very little from the spherical 

 figure. In fishes it is flattened on its an- 

 terior part ; in birds it is remarkably 

 convex in front, the cornea being some- 

 times absolutely hemispherical. The 

 convexity of the crystalline is an inverse 

 ratio to that of the cornea. Thus in fishes 

 it is nearly spherical, and projects 

 through the iris, so as to leave little or 

 no room for aqueous humour : the ceta- 

 cea, and those quadrupeds and birds 

 which are much under water, have this 

 part of the same form. The aqueous hu- 

 mour, being of the same density with the 

 medium in which these animals are plac- 

 ed, would have no power of refracting 

 rays of light which come through that 

 medium : its place is supplied by an in- 

 creased sphericity of the lens. In ;>irds 

 these circumstances are reversed : they 

 inhabit generally a somewhat elevated 

 reigon of the atmosphere, and the rays 

 which pass through this thin medium are 

 refracted by the aqueous humour, which 

 exists in great abundance. Man, and the 

 mammalia, which live on the surface of 

 the earth, hold a middle place between 

 these two extremes. 



The inner surface of the choroid coat, 

 which in man is black throughout, is co- 

 loured very beautifully on the temporal 

 side of the eye in most quadrupeds, and 

 this part is called the tapetum. 



The pigmentum nigrum is entirely de- 

 ficient in the eye of the white rabbit, 

 white ferret, &c. as well as in the va- 

 riety of the human race called the al- 

 bino. 



The quadrumana alone possess the 

 foramen centrale of the retina, besides 

 snan. 



Most mammalia possess a membrana 

 nictitans, or third eyelid, behind which 

 the eyeball can be drawn, when offended 

 by any extraneous matter. 



Birds are distinguished by having a 

 bony ring, composed of numerous flat 

 and over-lapping thin plates, in the sub- 

 stance of the sclerotica, at its anterior 

 part. 



Another great peculiarity consists in 

 the marsupium or pecten, which ap- 



pears as a large folded process of the 

 choroid, coming through the retina of 

 the back of the eye, and running in the 

 substance of the vitreous humour towards 

 the crystalline lens, which it does not 

 quite reach. 



The third eyelid, or membrana nicti- 

 tans of birds, is a thin semi-transparent 

 fold of the conjunctiva ; which, in the 

 state of rest, lies in the inner corner of 

 the eye, with its loose edge nearly verti- 

 cal, but can be drawn out so as to cover 

 the whole front of the globe. By this, ac- 

 cording to Cuvier, the eagle is enabled to 

 look at the sun. 



It is capable of being expanded over 

 the globe of the eye by the combined ac- 

 tion of two very singular mus-les, which 

 are attached towards the back of the scle- 

 rotica. One of these, which is called 

 from its shape the quadratus, arises from 

 the upper and back part of the si ieroti- 

 ca ; its fibres descend in a parallel course 

 towards the optic nerve, and terminate in 

 a semicircular margin, formed by a ten- 

 don of a very singular construction ; for 

 it has no insertion, but constitutes a cy- 

 lindrical canal. The second muscle, which 

 is called the pyramidalis, arises from the 

 lower and back part of the sclerotica to- 

 wards the nose. It gives rise to a long 

 tendinous chord, which runs through the 

 canal of the quadrat us, as in a pulley. 

 Having thus arrived at the exterior part 

 of the eyeball, it runs in a cellular sheath 

 of the sclerotica along the under part of 

 the eye to the lower portion of the loose 

 edge of the membrana nictitans, in which 

 it is inserted. 



By the united action of these two mus- 

 cles, the third eyelid will be drawn jto- 

 wards the outer angle of the eye, so as to 

 cover the front of the globe ; and its own 

 elasticity will restore it to its former situ- 

 ation. 



Two kinds of eyes, very dissimilar in 

 their structure, are found in insects : one 

 sort in small and simple, (stemmata ;) the 

 others, which are large, seem to consist 

 of an aggregation of smaller eyes ; for 

 their general convexity is divided into an 

 immense number of small hexagonal con- 

 vex surfaces, which may be considered 

 as so many distinct corneae. The first kind 

 is found in different numbers in most of 

 the aptera, as also in the larvae of many 

 winged insects. When these undergo 

 the last or complete metamorphosis, and 

 receive their wings, they gain at the 

 same time the large compound eyes. 

 Several genera of winged insects and ap- 

 tera (as the larger species of monoculi,) 



