THE ACCMSSOST ORGANS OF VISION. 835 



it passes to the inner surface of the outer wing of the nostril ; there it 

 terminates by an orifice, sometimes two, that looks as if punched out of the 

 membrane, towards the lower commissure, near the point where there is a 

 line of demarcation between the dark colour of the skin, and the rosy tint 

 of the mucous lining. 



" This aperture constitutes the ' nasal outlet ' (egout nasal). 



"The epithelium of the membrane lining the canal is ciliated in its 

 bony, stratified in its nasal, portion. On the surface of the membrane are to 

 be seen the openings of the secretory ducts of some conglomerate glands, 

 which are lodged in the walls of the canal. Throughout its extent, the 

 canal is lined by a continuation of the mucous membrane of the lachrymal 

 sac. In Solipeds, this canal opens on the cutaneous surface at the entrance 

 of the nostrils ; and it therefore happens that in these animals the con- 

 junctiva, with its dependencies, forms a particular mucous membrane, really 

 apart from the great gastro-pulmonary membrane. 



'_' In the Ass and Mule, the orifice of the lachrymal canal is situated at 

 the inner face of the outer wing of the nostril, and not near the inferior 

 commissure, as in the Horse." — F. Lecoq : ' Exterieur du Cheval,' etc. 



(Sometimes this outlet is double. The lachrymal secretion is not only 

 useful in facilitating the movements of the eyelids over the eyeball, but it 

 washes away dust and hurtful matter from off the surface of the cornea, 

 keeping the latter clean, moist, and healthy.) 



DrtTEBENTIAL CHAEACTEBS IN THE VISUAL APPAEATtJS IS OTHEE THAN SOI.IPED 



ANIMALS. 



Essential Organ or Vision. — In the Ox, the eyeball resembles in shape that of tho 

 Horse ; but in small animals, particularly the Vog, it is much more spherical. In Birds, 

 it is very convex in front ; its largest diameter is the antero-posterior. 



Sclerotic. — This is the same in all the domesticated quadrupeds. In Birds, however, it 

 has some curious features. Posteriorly, it has for base a cartilaginous layer, covered on 

 both sides by fibrous tissue ; this layer frequently ossifies around the optic nerve, where 

 it forms the posterior sclerotic ring. Around the cornea, there is the anterior sclerotic 

 ring, composed of small bony imbricated scales, capable of moving on each other, and 

 modifying the shape of the globe of the eye. 



Cornea. — In the Dog and Cat, the structure of the cornea is similar to that of the 

 Horse. (Kolliker states that he observed lymphatics in the cornea of a young Cat.) _ In 

 the Ox, Sheep, and Pig, there are two limitary meriibranes ; one beneath the epithelium 

 of the anterior face, in Birds, this limitary membrane is thickest in front. 



Choroid.— la mamniifers, there are some slight diiferences in the coloration of the 

 tapetum. Thus, in the Ox. it is gulden green, which becomes blue at the circumference ; 

 in the Sheep, it is a pale golden green ; a golden yellow in the Cat ; and white, bordered 

 with blue, in the Bog. (It is absent in the Pig.) In Bird^, it is uniformly black ; this 

 membrane has also a network of non-striped muscular fibres, and, in addition, •' Crampton's 

 muscle, which arises from the inner face of the osseous ring, and is inserted into the 

 cornea." — Leydig. (According to Hassenstein, in rapacious animals there is, behind the 

 tapetum, a layer of corpuscles composed of lime salts; to this is owing the brilliancy of 



their eyes in the dark.) . . t .i, . ,-, n, 



Zris.—ln all animals the iris is muscular. In mammifers, the contractile fibres are 

 non-striped ; in Birds, they are striped. (In the Ox, its anterior face has a brighter 

 colour than in the Horse. In the Sheep it is a brownish-yellow ; in the Goat blue.) In 

 the Dog its colour is a more or less bright golden-yellow ; in the adult Cat green ; and 

 in yoimg animuls a bright blue. The pupil is elliptical in the Ox, as in Solipeds (in 

 the Sheep and Goat it is more elongated) ; in the Dog it is circular, and, when very 

 much dilated, it is the same in the Cat; but, when contracted, it becomes elliptical 

 vertically, and may be so narrow as to represent nothing more than a thin perpendicular 

 slit (In the Pig it is round.) 



There are no diiferences worthy of note in the otlfer parts of the eye. 

 AccESSOET Oeoans OF THE VISUAL AppAEATus. — The motor organs are nearly the 

 same in all the other animals. 



