Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/742

724 rudiment. Hence the Ostrich has only two toes (which answer to the third and fourth of the pentadactyle foot), with four phalanges in the inner and five in the outer, though the inner toe is far the longer and the stronger. In most four-toed Birds the hallux is turned more or less completely backwards, and the other three digits forwards. But in many Aetomorphce (especially the Owls) the outer toe can be turned outwards, or even backwards, at will. And in the Parrots, Toucans, Cuckoos, Woodpeckers, and other so-called &quot; Scansorial &quot; Birds, the outer toe is per manently reversed. Under these circumstances the distal end of the outer raetatarsal may be divided into two dis tinct articular surfaces. In the Trogons there are two toes in front and two behind, as in the Parrots ; but it is the second toe which is turned backwards. Lastly, in the Swifts, the Dysporomorphce, and the Spheniscomorphce, the hallux is directed more or less forwards, so that all four toes are turned to the front. As a general rule, the osseous tissue of Birds is remark ably dense and hard. Before hatching, the bones are solid and filled with vascular medulla ; but after birth, more or fewer of the bones are always excavated by prolongation of cavities containing air, which lie in their neighbourhood. Such air cavities are always found in the skull, in connec tion with the nasal and auditory passages, and they may extend through all parts of the skull, with the exception of the jugal arch, which, however, is pneumatic in the Toucan and Hornbill. In many birds, Apteryx, Penguin, Divers (and Gulls, according to Professor Huxley ; but this is a mistake, their spinal column far into the sacrum is pneumatic ; Larus canus shows this well), and the smaller Song-birds, no other bones than those of the skull are pneumatic ; but in most birds the air-sacs of the lungs send prolongations into the bones of the rest of the trunk- skeleton, seldom into the caudal vertebras, as in Baloeniceps, the Adjutant, Hornbill, &c. In the Hornbills the whole skeleton is pneumatic ; in a large number of birds the humerus alone of the limb-bones contains air ; in the diurnal Raptores, the femur also. It is proper to remark that the amount of pneumaticity of bones by no means follows the development of the powers of flight. In the Ostrich, for example, the bones are far more extensively pneumatic than in the Gull. In some cases, prolongations of the air-sacs extend beneath the integument.

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In the space allotted to the writer, there is merely room for justice to be done to one category of organs ; and as the skeleton, and especially the skull, is of most direct import ance to the zcologist and palaeontologist, and as its form determines, as it were, all other organs, they being correlated with it and answering to it, it seemed to be that on which election should fall for the fuller treatment. An impartial description of all the systems of organs would have resulted in the merest outline for each. For the muscles, Professor Huxley s abstract must serve. The cutaneous muscles of Birds are well developed, and form broad expansions in various parts of the body. Special bundles of muscular fibres pass to the great quill feathers of the tail and wings, and others to the palagium, a fold of integument which stands between the trunk and brachium behind and between the brachium and ante-brachium in front. In correspondence with the slight mobility of the dorsal vertebrae, the episkeletal and hyposkeletal muscles of the spine attain a considerable development only in the neck and in the tail. Owing to the great size of the sternum, the abdominal muscles are usually small, and the internal oblique may be absent. A diaphragm, consisting of bundles of muscular fibre, which pass from the ribs to the aponeurosis, covering the ventral face of the lungs, is developed in all Birds, but attains to the greatest degree of completeness in the Ratitce, and especially in Apteryx. The muscles of the limbs are remarkably modified by the excessive development of some of those found in other Vertebrata, and the suppression of others. Thus in all birds possessing the power of flight, the pec- toralis major, the chief agent of the downward stroke of the wing, is very large and thick, taking its origin from the whole length, and a great part of the depth, of the keel of the sternum. The elevation of the wing is chiefly effected by the pectoralis secundus (levator humeri ; or p. medius, Macg., plate 3, figs. 4, 5), which arises beneath (within and over, in the standing bird) the foregoing muscle, and passes over the inner side of the scapulo-coracoid articulation as over a pulley, to reach the humerus. The muscles of the fore-arm and digits are reduced, in accordance with the peculiar modification of the skeleton of these parts. In the hind limb of most birds there is a singular extensor muscle, which arises from the pubis, and ends in a tendon which passes to the outside of the knee-joint and terminates in the leg by uniting with ihe flexor digitorum perforatus. The result of this arrangement is that the toes are flexed whenever the leg is bent upon the thigh, and consequently the roosting bird is held fast upon his perch by the weight of his own body.

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In Birds, as in Reptiles, the cerebro-spinal axis is angu- lated at the junction of the spinal cord with the medulla oblongata, the latter being bent down towards the ventral side of the body. The region on which the nerves of the anterior and posterior extremities originate is enlarged in Birds. In the lumbar enlargement the posterior columns of the cord diverge and give rise to the sinus rhomboidalis, which is a sort of repetition of the fourth ventricle, the dilated central canal of the spinal cord being covered merely by a thin membrane, consisting chiefly of the ependyma and arachnoid. The brain fills the cavity of the skull, and presents a well-developed cerebellum ; a mesencephalon, divided above into two optic lobes ; and relatively large prosencephalic hemispheres, which attain a considerable size but never conceal the optic lobes. The transverse fissures of the cerebellum are distinct, and the lateral appendages of the cerebellum, or flocculi, become well defined, and are wedged, as in many of the lower Mammalia, in cavities of the side walls of the skull, arched over by the anterior vertical semicircular canal. There is no ports Varolii, in the sense of transverse fibres connecting the two halves of the cerebellum, visible upon the ventral surface of the mesencephalon. The optic lobes contain ventricles ; these are thrown down to the sides of the base of the brain, and are connected over the aqucedudus Sylvii by a broad commissural band. Each prosencephalic lobe contains a lateral ventricle (continuous through the 