946 THE SENSORY APPARATUSES. 



(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 epithelium 

 clean, moist, and healthy.) 



Differential* Characters in the Visual Apparatus of the other Animals. 



t 



Essential Organ of "Vision.— In the Ox, the eyeball resembles in shape that of tlie 

 Horse ; but in small animals, particularly the Dog, 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. 

 In the Ox, Sheep, and Pig, there are two limitary membranes ; one, consequently, beneath 

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



Choroid. — In Mammifers, there are some sliglit differences in the coloration of the tapetum. 

 Thus, in the Ox, it is golden green, wliich becomes blue at the circumference; iu the Sheep, 

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

 Dog. (It is absent in the Pig.) In Birds, 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.) 



Jris. — In 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 young animals, 

 a bright blue. The pupil is elliptical iu the Ox, as iu Solipeds (in the Sheep and Goat, it 

 is more elongated) ; in the Dog, it is circular, and, when very much diluted, 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 differences worthy of note in the other parts of the eye. 



Accessory Organs of the Visual Apparatus. — The motor and protective organs 

 are nearly the same in all the other animals. 



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. 



(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.) 



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 suflBciently extensive to cover the entire front of the eye, and is moved by a curious little 

 apparatus. 



Glands.— In Ruminants, the Pig, and in Birds, there is found annexed to the membrana 

 nictitans, Earder's gland— & conglomerate gland, with adipose epitlielium 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 is 

 voluminous; it has two large and several small ducts. The lachrymal gland is also voluminous, 

 and its nasal opening is situated liigher in the nostril than in 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.) 



