In no case does any bird, even Archæopteryx, possess a fifth toe. Unlike mammals, the number of joints in the toes varies greatly in birds. In those which possess four toes we find the following number of joints: in the first, two; in the second, three; in the third, four; and in the fourth, five. This rule holds for nearly all birds, but the Swifts have never more than three joints, and in the Goat-sucker and the Sand Grouse there are two less than ordinary on the fourth toe. In a number of birds the inner toe (big toe of man) disappears, and in the Ostrich proper the next division of the “typically” five-toed foot, or second toe, has no toe-joints.
In dealing with the muscular system of birds, we need here concern ourselves with only those special muscles which are modified in accordance with the necessities of the bird’s habits, and those other muscles which have been brought into special notice by valuable investigations.
That great fleshy mass which is found on the breast of a bird, and which is not unknown to those who are fond of a good “dish,” consists of three separate muscles, two of which depress, while the other elevates the wing. The presence of the elevator muscle on the lower side of the sternum is a curious arrangement by which the centre of gravity of the animal is lowered—a most necessary condition in flight; the tendon from this muscle passes through a pulley-like canal to be inserted into the upper side of the head of the bone, which, as has already been explained, is known as the humerus, so that when it contracts it draws this bone up. The ability of the wings to resist the pressure of the air is clearly dependent on the power possessed by these muscles. Borelli has calculated that the “pectoral muscles” of the bird exceed in weight all the other muscles taken together, whilst in man the pectoral muscles are but a seventieth part of the mass of the muscles.
The large and important muscles, which in the Mammalia, constitute the diaphragm, or midriff, are ordinarily said to be absent in birds, and, indeed, in most cases are but feebly represented. In the Ratitæ, and especially in the New Zealand form (Apteryx) of this group, the diaphragm may attain to a very fair degree of completeness, though even here the apex of the heart is allowed to pass into the abdominal cavity. The muscles of the back are feebly developed, as might be imagined from the firm character of the spinal column; and as the fore limb exhibits but slight power of varying its movements, its muscles are not well developed. Those muscles which are found in the skin are, on the contrary, expanded into broad pieces; and special bundles are sent to the larger feathers of the wings and of the tail, and to those folds of skin which connect the upper arm with the trunk, and with the fore arm, respectively. Borelli thus explains the arrangement by which a perching bird remains fixed when asleep: A muscle which arises from the pubes bone of the hip-girdle passes over the knee, and then takes a backward direction so as to pass behind the ankle; it thus becomes one of the flexor muscles, by the contraction of which the toes are flexed, or bent. When the perching bird, which, as we know, has one of its toes directed backwards, is seated on a bough, the thigh has its upper end directed backwards, while the upper joint of the leg is turned forwards, or in other words, the two parts of the leg have opposite directions. This arrangement acts as a contracting influence on this muscle and its tendons, while the weight of the bird is sufficient to preserve this condition and the consequent flexion of the toes.
To turn to those muscles the arrangement of which has, been made the basis of a suggested classification. In the leg of the bird there are, among others, four muscles, the names of which are femoro-caudal, accessory femoro-caudal, semi-tendinosus, and accessory semi-tendinosus, any of which may be absent, but in those cases where a single muscle only is found the first is always present; again, there is a muscle which, from its course, is known as the ambiens, and this, too, may be present or may be absent. As the presence or absence of any of these muscles is a very constant phenomenon in any given section of birds, it has been proposed to divide the class into those which do, and those which do not, possess the above-named ambiens muscle. In the latter group the second of the four above-named muscles—the accessory femoro-caudal—is never present.[141]
Of all the muscles, those which act in aid of the vocal organs are of the greatest interest, but they will be considered a little later on.
A valuable suggestion has been made, which, if followed out, may lead us to understand how it is that the brain of the bird, which is so simple as compared with that of man, is nevertheless capable of so much intelligent activity. Bearing in mind the axiom that it is quality not quantity that tells, and looking at the fact that the brain of the most highly intelligent man is, after death, supposed to be similar to that of the foolish and of the unwise of our race, it is obvious that the essential difference must lie elsewhere than in the coarser, or more evident, characters of that organ which is known as the brain. The suggestion, then, that was made, was to the effect, that the possessors of aviaries, in which it was possible to study the characters of birds, should submit the brains of their deceased favourites to that more thorough investigation which the microscope allows of. The brains of birds vary but little in their anatomy. The optic lobes are rounded, paired, and tubercular in the bird, and are not divided into four, as in mammals; they are found at the lower part and sides, and not in the upper part of the brain. The cerebellum is not continued at the sides into distinct lobes; nor are the two lobes of the brain (or cerebral hemispheres) provided with those convolutions which, in mammals, seem to increase in complexity of character as the animal rises in the scale of intelligence. The cerebrum does not cover the cerebellum. Small as is the brain of birds, it is found that, in many, its weight is, as compared with that of the body, much greater than it is in man.
With reference to the spinal cord, or the continuation of the central part of the nervous system through the vertebral column, it is only necessary to remark that it is much increased in width at the two regions, in which the nerves for the fore and hind limbs are respectively given off; that there is a narrow canal running along its centre, and that at the lower end there is a large space. In regard to the cerebral nerves, those for the eyes are of great size.
Coming now to consider the organs of the senses, and beginning with the eye, it is interesting to note that there are no blind birds, and, indeed, the eyes are of a large size as compared with the brain. They are generally placed at the sides, though the nocturnal birds of prey (in which they are directed forwards) are an exception to this rule. It is in very rare cases that eyelashes are present, and although they seem to exist in the group just mentioned, it is probably more correct to look upon them as slightly modified feathers.
If the eye be regarded as having on its front face, a part which would, if completed, form part of a smaller circle than the rest of the eye, it is clear that this cornea, or front part, would be more convex than the rest, and that it would consequently be a “more powerful glass,” inasmuch as it would exert a greater bending (refracting) influence on the rays of light which pass through it, while, further, it is clear that the more convex it is the better “glass” would it be. Now this is just what happens in birds: the cornea is very convex; in addition to this, the long axis of the eye, on the length of which it seems that, in many cases, the condition known as that of being “short-sighted” depends, is very long in some birds, and notably in the Owls.