474 SPECIAL PHYSIOLOGY. 



presents a greenish-blue metallic lustre ; in most Carnivora, it has a silvery 

 hue, excepting in the cat, in which it is green ; sometimes it resembles mother- 

 of-pearl. The Quadrumana, Edentata, and Monotremata, like man, have no 

 tapetum. The shape of the pupil also presents peculiarities in certain Mam- 

 malia ; thus, in the cat it is elliptical, and even linear, in a perpendicular di- 

 rection, when contracted under a strong light. Solipeds and Kuminants have 

 a transversely oblong pupil. It is remarkable that the yellow retinal spot is, 

 as a rule, absent in Mammalia, the only exceptions being in the Quadrumana. 



In Birds, the eyelids are well developed, and, except in owls and a few 

 others, the lower lid is generally more movable than the upper one ; contrary 

 to what occurs in Mammalia, it covers the larger part of the eyeball, and is 

 even provided with a special depressor muscle. The third eyelid, or mem- 

 brana nictitans, is always present, and fully developed ; in birds of prey, it is 

 in constant use, serving to cleanse the eyeball, or to temporarily diminish the 

 glare of bright sunlight ; it is sometimes nearly transparent, but usually 

 rather opaque. The nictitating membrane folds back on the side next the 

 nose, by the action of its elastic tissue ; but, for its projection over the front 

 of the eyeball, two muscles are provided. Of these, one, named the pyrami- 

 dalis, is a slender muscle, arising from the sclerotic, passing behind the back 

 of the eyeball,' curving over the optic nerve, and ending in a tendon, which 

 slides through the border of the other muscle. This muscle, named the quad- 

 ratus, descends from the upper part of the eyeball, and forms a muscular 

 pulley for the tendon of the pyramidalis. After escaping from the pulley, 

 this tendon continues over the back of the eyeball, and finds its way to the 

 lower border of the nictitating membrane. The contraction of the pyrami- 

 dalis pulls the membrane across the eyeball ; whilst the quadratus prevents 

 the tendon of the muscle from straightening itself, and so coming down upon 

 the optic nerve. Birds have, in addition to the ordinary lachrymal gland, a 

 large Harderian gland, situated behind the conjunctiva, at the nasal angle of 

 the eyelids. The muscles of the eyeball are the same as in Mammalia ; the 

 superior oblique does not, however, pass through a pulley. 



The eyeball of the bird is very large in comparison with the size of the head 

 and brain, especially in the nocturnal birds of prey. It is not usually so 

 spherical as in Mammalia, but is sometimes somewhat flattened, and, in the 

 larger raptorial birds, is often lengthened by the prolongation of the anterior 

 part of the sclerotic, and by the extremely convex form of the cornea. Mov- 

 able bony plates, situated in the sclerotic, frequently preserve this elongated 

 form, an arrangement also found in certain reptiles and fishes, though in 

 them, the eyeball is flattened. The choroid coat in birds generally, sends for- 

 wards into the vitreous humor, from near the entrance of the optic nerve, a 

 remarkable plicated vascular membrane, named the pecten, or marsu.pium. 

 This is falciform, or irregularly quadrangular in shape ; its plicse, or folds, are, 

 in some species, only four, but, in others, nearly thirty in number. The pec- 

 ten sometimes reaches the posterior part of the capsule of the lens. It is not 

 muscular, but is supposed, by means of changes in the state of distension of 

 its vessels, to alter, either directly or indirectly through the vitreous humor, 

 the position of the lens in the interior of the eyeball, and to assist in adjusting 

 the focal distance of the eve. The action of the pecten, which varies in size 

 and shape in different birds, is, however, not well understood. The Apteryx 

 australis is the only bird in which the pecten is absent. The iris is usually 

 very active, and contains striated, as well as unstriped, muscular fibres ; its 

 movements are more active and rapid than in other animals, in which it con- 

 tains only plain muscular fibres ; and, in some birds, it is said to be even 

 under the influence of the will ; its color varies, but it never exhibits a metal- 

 lic lustre. The pupil is generally round, though it is lengthened vertically in 

 the owl, and horizontally in the dove and goose. The internal ciliary muscle 

 also exists, and doubtless influences the form and position of the lens ; whilst 

 the muscular fibres around the circumference of the cornea (Crampton), and 

 even the proper muscles of the eyeball, may, by compressing its movable os- 

 seous plates, alter the relative convexity of the cornea, and so assist in focus- 

 sing the images of objects upon the retina, however variable their distance. 

 The power of adjusting the eye to accurate vision at different distances, exists 



