836 THE APPARATUS OF THE SENSES. 



(The posterior rectus, or retractor muscle, is most developed in Ruminants, which, 

 during their whole time of feeding, have the head in a dependent position. In most of 

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

 there are four distinct strips, almost resembling a second set of recti muscles, but deep- 

 seated, and inserted into the posterior, instead of the anterior, portion of the globe.) 



Muscles. Birds have only six muscles: four recti, and two oblique. The latter 

 arise from the anterior wall of the orbit ; consequently, the great oblique does not pass 

 through a pulley. 



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

 lid is the largest, and 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. 



Glands. In Ruminants, the Pig, and in Birds, there is found, annexed to the mem- 

 brana nictitans, Harder' s 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, doubtless, to 

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

 eyelids. (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 bone, 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 arch ; in the Cat, this ligament is 

 smaller, and the orbital process of the zygomatic concurs with that of the frontal bone 

 to form the upper wall.) 



COMPARISON OF THE VISUAL APPARATUS OF MAN WITH THAT OF ANIMALS. 



ESSENTIAL 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 Solipeds. 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 & of an inch in 

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

 lutea), with the fossa centralis of the retina (fovea centralis, foramen of Soemmering). 



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 to be con- 

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

 of. the eyeballs 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 ORGANS OF THE VISUAL APPARATUS. The orbital cavity of Man is 

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

 fascia has been described as separating the eye from its surrounding adipose tissue, and 

 which, like a "tunica vaginalis," enables the globe to roll with rapidity and precision.) 

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

 same as in animals. Only the rudiment of a caruncida lachrymalis is present. The 

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



