The eye is covered in by a firm and strong membrane, which is known as the “sclerotic;” this, in its front part, develops a number of bony plates; of these there may be as many as twenty, and they are capable of a certain amount of free movement on one another. What is known as the power of accommodation depends upon the extent to which the front face of the somewhat lens-shaped body which helps to separate the eye into two chambers is capable of being rendered more or less flat; this front face is covered by a membrane which is found to be more or less taut, according to the state of contraction of the muscles (ciliary muscles) connected with it. A very little reflection is sufficient to show that a swiftly moving animal has the focus of its eye, or the point at which clear vision is alone possible, changed much more rapidly than an animal which moves more slowly. So much on the one side. On the other, it is to be observed that muscles vary in structure; they are either “smooth” or “striated,” and it is the latter that contract the more rapidly. Putting these two series of observations together, it is easy to arrive at the result that a bird should have striated muscular fibre in its ciliary muscles, and a more slowly moving animal like man, smooth muscular fibres; and this we find to be the case! The iris is an arrangement by which the quantity of light admitted into the eye is enabled to be varied, and the small hole in the centre, through which the rays of light pass, is known as the pupil; this is always rounded in birds, and is never elongated as it is in some mammals—the Cats, for example.[142]
But the most peculiar arrangement in the bird’s eye is the presence, projecting into the hinder chamber, of a membrane in which run blood-vessels; this, which is known as the pecten (comb), or marsupium (pouch), enters the vitreous humour, which fills up this hinder chamber by the same cleft as the optic nerve. It is folded, and is generally of a quadrangular shape; it is not found in the eye of the Wingless Bird of New Zealand (Apteryx).
SECTION OF THE EYE OF THE COMMON BUZZARD.
(After Macgillivray.)
(aa) Sclerotic; (bb) Choroid Coat and Pigment; (c) Ciliary Circle; (d) Lens; (ee) Iris; (f) Cornea; (g) Optic Nerve; (i) Pecten.
A third eyelid is well developed in this class; it is an elastic membrane (membrana nictitans, or winking membrane), which has not, like the other two, a vertical movement, but is drawn obliquely over the eye from the inner to the outer side. This movement is effected by two special muscles, one of which arises on the inside, and below the eyeball, and has therefore to pass over to the outer side. In contracting, it would press on the optic nerve, were it not for the other one, which, however, is so disposed that by its contraction it draws away the tendon of the pyramidalis muscle from pressing on the nerve. As in ourselves, there are six special muscles for moving the orbit or ball of the eye, but the one which in man is well enough known as the trochlear, has no pulley-arrangements in birds. Lachrymal glands are present.
With regard to the organ of hearing, one particular part, which in man is in the form of a snail’s shell, and is known as the cochlea, is not coiled into this shape in birds, being very slightly bent, though holding in other respects the same general relations. Nor is there any external ear, as in mammals, for collecting the waves of sound; there is, however, in the nocturnal birds of prey a crescent-shaped valve on which are set tufts of short feathers, and it is possible that this may aid in hearing. Nor, again, are there in the interior of the ear those three small bones, which are known generally as the auditory ossicles; of the two that are absent, one is thought by many anatomists to be represented by the quadrate bone, which, as has already been mentioned, connects the lower jaw of the bird with the skull. The single bone which is present, and which is, perhaps, most generally known as the “columella,” is connected by two or three cartilaginous processes with the drum of the ear, and by the other end—at which it has a small oval plate—with the more internal parts of the organ of hearing. In man there is a curious arrangement of rods, which vary in so remarkable a way as to have led to the supposition that each was adapted to a distinct note; these rods, which constitute the organ of Corti, are not present in birds, affording thereby a striking example of the law that physiological inferences are often well examined by the aid of comparative anatomy, no physiologist being hardy enough to deny to birds the power of appreciating those delicate modulations of sound which go to make up the chief charm of music. With regard to the organ of smell, it is only necessary to note the absence of those muscles by which, in man and other mammals, the nostrils are contracted or dilated.
The first point which attracts us on examining the digestive tract of birds is the absence of lips and of teeth; but with regard to these latter we must note that it is a character which has only become distinct since the time when birds were first developed. This statement is borne out by two series of facts, each taken from one of the two great aids to a correct apprehension of the real importance of structural characters—that is, from embryology, or the study of the developing individual; and from palæontology, or the natural history of the past. The young of certain Parrots have been observed to possess, at an early stage of their development, those uprisings on the mucous membrane of the jaw which go by the name of “dental papillæ,” and these papillæ have been seen to be covered with a cap of dentine. On the other hand, the researches of Owen and of some American palæontologists have brought to light bird-like forms which were provided with teeth (Odontornithes: Ichthyornis, Hesperornis).
The beak, or horny covering of the jaws, varies very greatly in form, and in the degree of its sensibility. This tactile sense is dependent on the extent to which the beak is supplied by nerves (from the fifth cerebral nerve). In the Woodpecker, for example, there is a large branch extending along the inside of the lower jaw, which, as it approaches the extremity, breaks up into finer nerves that perforate the bone by a number of small canals and so give to the beak a power of discovering what lies hid in the crevices of the wood and under the bark. Being an external structure, the beak is naturally adapted to the habits of its possessors, so that it may be hooked, as in many flesh-eating forms, or trenchant, and fit to cut and break, or provided with transversely-set fine plates by which the water taken in with the food can be filtered off, or provided with bristles, the better to hold a living prey. Finally, in many cases the hardness of the bill is made up for by a patch of naked skin at the base of the upper mandibles, which is known by the name of the “cere” and seems to have a tactile function.
In many birds, the tongue is either feebly developed, or is encased in horn, so that it can hardly be as useful an organ of taste as is our tongue: in the Pelicans it is obsolete. In some, however, as in the Woodpecker, the tongue is a very powerful seizing organ, as it is protruded with great rapidity by means of a special muscle, and is well provided with a sticky secretion, which is given off from a large gland (the sub-lingual), which, lying below the muscle above referred to, is compressed when this muscle contracts; so that in the Woodpecker, just as in the mammal called the Great Ant-eater (Myrmecophaga), the insect prey is easily captured.