Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/414

Rh 392 MAMMALIA [CETACEA. together to form a sacrum. The lumbar and caudal vertebrse are numerous and large, and, as their arches are not connected by any articular processes (zygapophyses), they are capable of a very free motion in all directions. The epiphyses at the ends of the vertebral bodies are very distinct flattened disks, not uniting until after the animal has attained its full dimensions. 1 There are largely developed chevron bones, the presence of which indicates the distinction between the caudal and lumbar vertebrae. The skull is modified in a very peculiar manner. The brain-case is short, broad, and high, almost spherical in fact. The supra-occipital bone rises upwards and for wards from the foramen magnum, to meet the frontals at the vertex, completely excluding the parietals from the upper region of the cranium. The frontals are expanded laterally to form the roof of the orbits. The anterior narial aperture opens upwards, and has in front of it a NO. Fr Pa corpora cavernosa are attached to them. In some species, to the outer surface of these are fixed other small bones or cartilages, the rudiments of the hind limb. Teeth are generally present, but exceedingly variable in number. In the existing species, they are of simple, uni form character, all having conical or compressed crowns and single roots, and are never preceded by milk teeth. They are therefore homodont and monophyodont. In one group, the Mystacocetes, the teeth are absent (except in the foetal condition), and the palate is provided with numerous transversely placed horny laminae or &quot; baleen.&quot; The salivary glands are rudimentary or absent. The stomach is multilocular. The intestinal canal simple, and only in some species provided with a small csecurn. The liver is very little fissured, and there is no gall-bladder. The vascular system is greatly complicated by arterial and venous plexuses, or retia miralilia. The larynx is of peculiar shape, the arytenoid cartilages and the epiglottis being much elongated, and together form ing a tubular prolongation, which projects into the posterior nares, and when embraced by the soft palate forms a continuous passage between the nostrils and the trachea, as in the Ungulates, but in a more perfect manner. The brain is large re latively to the size of the animal, very round in form, and with its surface divided by sulci into very numerous and complex convolutions. The kidneys are deeply lobulated. The testes are ab dominal. There are no vesiculse semiuales, nor os penis. The uterus is bicornuate, the placenta non- deciduate and diffuse. The mammae are two in number, and the nipples placed in depressions on each side of the vulva. The principal ducts of the gland are dilated during lactation into large reservoirs, into which the milk collects, and from which it is injected by the action of a compressor muscle into the mouth of the young animal, by which means the process of sucking under water is greatly facilitated and expedited. The animals of the order Cetacea abound in all known seas, and some species are inhabitants FIG. 42. A Section of the Skull of a Young Dolphin (Gtobicephalus melas). x J. PMx, pre- t f i loi-o-pr rivprs nf Rnntli Amprira ami A^in maxiLa; MX, maxilla; ME, ossified portion of the mescthmoid ; an, anterior nares; Na, 01 U America ailU ASia. nasal; IP, inter-parietal ; Fr, frontal ; Pa, parietal ; SO, supra-occipital; ExO, ex-occipital; Their Organization necessitates their passing their BO, basi-occipital ; Sg, sqnamosal : Per, pcriotic ; AS, alisphenoid ; PS, presphenoid ; Pt, ,.. ,. D,. ,-, i j J.T i pterygoid; pn, posterior nares; PI, palatine; Vo, Vomer; s, symphysis of mandible; id, life entirely in the Water, as Oil Jand they are &quot; K ExO nferior dental canal ; cp, coronoid process of mandible ; cd. condyle ; a, angle ; sh, stylo-hyal ; liitelv helpless They have however to rise Very Mi, basi-hyal ; t/i, thyro-hyal. From Osteology of Mammalia. r ^, / / ^ frequently to the surface tor the purpose ot respira- niore or less horizontally prolonged rostrum, formed of tho maxillae, premaxillse, vomer, and mesethmcid cartilage, ex tending forwards to form the upper jaw or roof of the mouth. There are no clavicles. The humerus is freely movable on the scapula at the shoulder-joint, but beyond this the articulations of the limb are imperfect, flattened ends of the bones coming in contact with each other, with fibrous tissue interposed, allowing of scarcely any motion. The radius and ulna are distinct, and about equally developed, and much flattened, as are all the bones of the manus. There are four, or more commonly five, digits, and the number of the phalanges of the second and third digits always exceeds the normal number in mammals, sometimes very considerably ; they present the exceptional character of having epiphyses at both ends. 2 The pelvis is repre sented by a pair of small styliform bones placed longitu dinally, suspended below and at some distance from the vertebral column at the commencement of the caudal region. These appear to represent the ischia, as the crura of the 1 This is an important distinction from the Sirenia, but a character common to nearly all other mammals. It is doubtful whether there is any foundation for the statement that these epiphyses remain ununited for an exceptionally long period in the Cetacea. 2 A character repeated in some of the Seals. tion; and, in relation to the constant upward and downward movement in the water thus necessitated, their principal instrument of motion, the tail, is expanded horizontally, quite unlike that of a Fish, whose movements are mainly in straightforward or lateral directions. The position of the respiratory orifice or nostril on the highest part of the head is very important for this mode of life, as it is the only part of the body the exposure of which above the sur face is absolutely necessary. Of the numerous erroneous ideas connected with natural history, few are so widespread and still so firmly believed, notwithstanding repeated expositions of its falsity, as that the Cetacea spout out through their blowholes water taken in at the mouth. The fact is, the &quot;spouting,&quot; or more properly &quot;blowing,&quot; of the Whale is nothing more than the ordinary act of expiration, which, taking place at longer intervals than inland animals, is performed with a greater amount of emphasis. The moment the animal rises to the surface it forcibly expels from its lungs the air taken in at the&quot; last inspiration, which of course is highly charged with watery vapour in consequence of the natural respiratory changes. This, rapidly condensing in the cold atmosphere in which the phenomenon is generally observed, forms a column of steam or spray, which has been erroneously taken for