s::<; I III: M'l'A I.'ATUS OF Till: 8BI 



(The pocterior rcctus, or retractor muscle, is most develojwd in Ruminants, which, 

 (hiring their whole time of feeding, have the head in a dependent position. In n 

 the Carnivora, instead of this muscle forming a complete hollow cone. as in Ruminants, 

 there are four dibtinct strips, almost resembling a i-eeoud act of recti muscles, but decp- 

 icated, and inserted into the posteiior, instead of the anterior, imrtion of tin- globe.) 



Muxcles. Birds have only six muscles: tour recti, and t\w> oblique. Tii. latter 

 arise from the anterior wall of tho orbit; consequently, the great oblique does u< 

 through a pulley. 



Eyelids. The disposition of these is the same in all mammifers. In Birds, the lower 

 lid is the largest, aud is furnished with a particular depressor muscle; there are no 

 Meibomian glands. There is a third eyelid, corresponding to the membrana nictitans of 

 quadrupeds; it is sufficiently extensive to cover the entire front of the eye. 



GUmds. In Ruminants, the Pig, and in liirds, there is found, annexed to the nifm- 

 brana nictitans, Harder** gland a. conglomerate gland, with adipose epithelium in 

 mammifers, and cylindrical and granular in Birds It secretes a thick white matter, 

 which is thrown out on the membrana by one or two orifices. Its use is, doubt ! 

 favour the movements of that organ over the surface of the eye, as well as those of the 

 eyelid*. (In the Ox, this gland and its ducts are large. The lachrymal gland is also 

 voluminous and its nasal opening is situated higher in the nostril than with the Horse. 

 In the Sheep, there are found, near the lachrymal fossa, several adipose follicles which 

 do not properly belong to this apparatus, and which secrete a consistent, unctuous, 

 yellow matter. In the Pig, the lachrymal ducts are separated, by a bony partition, 

 into two sets, as far as the lachrymal sac.) 



(Orbital cavity. In Ruminants, the frontal and superior extremity of the maxillary 

 bones contribute largely to the formation of this cavity. In the Pig, the upper part of 

 the orbit is not completed by the orbital process of the frontal Iwnc, which is short ; it is 

 continued by a ligament. In the Dog, the superior portion of the cavity is entirely 

 formed by a ligament, which replaces the orbital nrch; in the Cat, this ligament is 

 smaller, and the orbital process of the zygomatic concurs with tl.at of the frontal bone 

 to form the upper wall.) 



COMPARISON OF THE VISCAL APPAUATVS OF MAN WITH THAT OF ANIMALS. 



ESSEVTIAL ORGAN OF VISION. The eyeball of MAN is almost spherical, as in the 

 Carnivora. The sclerotic does not differ much. The cornea has two limitary membranes, 

 and is much less elliptical than in ^olipeds. The choroid has the same zones as in 

 animals; it is uniformly brown. The ciliary processes, 'seventy to eighty in number, are 

 a little longer than in the Horse, and do not exceed, in front, the ciliary ligament, to the 

 inner face of which they adhere throughout their external border. The pupillary opening 

 of the iris is always round. The retina is the same in structure as already described! 

 A little above the optic papilla, there is a circular or oval patch, about 5 ' T of an inch in 

 diameter, in the centre of which is a tiansparent spot ; this is the yellow spot (macula 

 lutea), with the/ott centralis of the retina (fovca centralis, foramen of Soemmeriiig). 



At this patch, the tissue of the retina is slightly modified, especially at the fossa; 

 there are only cones in the columnar layer, and all the other layers appear t> 1. 

 founded into one granular mass. (This spot only exists in animals which have tic 

 of the eyehalld parallel with each other, as in Man, the Quadrumana, and some .saurian 

 reptiles.) 



There is nothing particular in the aqueous humour, lens, or vitreous humour. 



ACCESSORY OKCANS OF THE VisrAL AI-PARATUS. The <>rl>. : t<il mriij of Man is 

 entirely inclosed by bony walls, and there is no fibrous sheath. (A fold of the orliital 

 fascia has been described as separating the eye from its surrounding adijKis. ti^ne, and 

 which, like a "tunica vaginalis," enables the globe to roll with rapidity and preei>ion ) 

 The muscles are six in number four recti, and two oblique; tho great oblique is the 

 same as in animals. Only tho rudiment of a caruncn'a larhrijntnli* is piv.-ent. Tho 

 nasal duct opens at some distance up on the surface of the infcr.or meatus. 





