Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/379

Rh SKELETON.] MAMMALIA 357 body of the vertebra, called the haemal arch. The last is, however, never formed in mammals by any part of the vertebra itself, but only by certain distinct bones placed more or less in apposition to it the ribs in the thoracic, and the &quot; chevron bones &quot; in the caudal region. In most cises the arch of one vertebra is articulated with that of the next by distinct surfaces with synovial joints, placed one on each side, called &quot; zygapophyses &quot; (2), but these are often entirely wanting when flexibility is more needed than strength, as in the greater part of the caudal region of long-tailed animals. In ad dition to the body and the arch, there are certain project ing parts called processes, chiefly serving for the attach ment of the numerous muscles which move the vertebral column. Of these two are single and median, the spi- nous process or neural spine, or ii M nricmio- frntn Fio.6. Side View of the First Lnmbar BIS g arisin irom vertebra of a Doc (Canis famHiaris). the middle of the Upper part of *, spinous process ; az, anterior ., j .1 i I zygapophysis ; pi, posterior zyg- tlie arch, and the hypapopliysis apophysis ; m, metapophysis ; a, from the under surface of the anapophysls ; ^ transverse process. body. This, however, is as frequently absent as the former is constant. The other processes are paired and lateral. They are transverse processes (t of which there maybe two, an upper and a lower, in which case the former is called, in the language of Owen (to whom we are indebted for the terminology of the parts of vertebrae in common use), &quot; diapophysis,&quot; and the latter &quot; parapophysis.&quot; Other pro cesses less constantly present are called respectively &quot;meta- pophyses &quot; (m) and &quot; anapophyses &quot; (a). The vertebral column is divided for convenience of description into five regions, the cervical, thoracic or dorsal, lumbar, sacral, and caudal. This division is useful, especially as it is not entirely arbitrary, arid in most cases is capable of ready definition ; but at the contiguous extremities of the regions the characters of the vertebrae of one are apt to blend into those of the next region, either normally or as peculiarities of individual skeletons. vical The cervical region constitutes the most anterior portion ,ebrae. o f the co i umnj or that which joins the cranium. The vertebrae which belong to it are either entirely destitute of movable ribs, or if they have any these are small, and do not join the sternum. As a gene ral rule they have a considerable perforation through the base of the transverse process (the vertebrar- terial canal (fig. 7, t), or, as it is sometimes described, they have two transverse processes, superior nnd inferior, which, meeting at their extremities, enclose a canal. This, however, rarely applies to the last vertebra of the region, in which only the upper trans verse process is USUallv de- FlG - 7. Anterior Surface of Sixth i i mi Cervical Vertebra of Dog. s, Veloped. 1 he transverse process, spinous process; az, anterior moreover, very often sends down S^f^ ^S^! near its extremity a more or less its inferior lamella. compressed plate (inferior lamella), which, being considered to be serially homologous with the ribs of the thoracic ver- tebrpe (though not developed autogenously), is often called the &quot; costal &quot; or &quot; pleurapophysial &quot; plate. This is usually largest on the sixth, and altogether wanting on the seventh vertebra. The first and second cervical vertebrae, called respectively &quot; atlas &quot; and &quot; axis,&quot; are specially modified for the function of supporting and permitting the free move ments of the head. They are not united together by the intervertebral substance, but connected only by ordinary ligaments and synovial joints. The cervical region in mammals presents the remarkable peculiarity that, whatever the length or flexibility of the neck, the number of vertebrae is the same, viz., seven, with only three known exceptions, the Manatee and Hoffman s Two-toed Sloth (Choice-pus hofmanni), which both have but six, and the Three-toed Sloth (Bradypus tridactyhis), which has nine, though in this case the last two usually support movable ribs, though not sufficiently developed to reach the sternum. The dorsal (or thoracic, as it would be more correctly Dorsal termed) region consists of the vertebrae which succeed those vertebra. of the neck, having ribs movably articulated to them. These ribs arch round the thorax, the anterior one, and most usually the greater number of those that follow, being attached below to the sternum. The lumbar region consists of those vertebrae of the Lumbar trunk in front of the sacrum which bear no movable ribs, vtrtebnr. It may happen that, as the ribs decrease in size posteriorly, the last being sometimes more or less rudimentary, the step from the thoracic to the lumbar region may be gradual and rather undetermined in a given species. But most commonly this is not the case, and the distinction is as well defined here as in any other region. As a general rule there is a certain relation between the number of the thoracic and lumbar vertebrae, the whole number being tolerably constant in a given group of animals, and any increase of the one being at the expense of the other. Thus in all known Artiodactyle Ungulata there are IS) dorso-lumbar vertebrae; but these may consist of 12 dorsal and 7 lumbar vertebrae, or 13 dorsal and 6 lumbar, or 1 4 dorsal and 5 lumbar. The smallest number of dorso-lumbar vertebrae in mammals occurs in some Armadillos, which have but 14. The number found in Man, the higher Apes, and most Bats, viz., 17, is excep tionally low 19 prevails in the Artiodactyles, nearly all Marsupials, and very many Rodents ; 20 or 21 in Carnivora and most Insectivora ; and 23 in Perissodactyla. The highest and quite exceptional numbers are in the Two-toed Sloth (Cholcepus) 27, and the Hyrax 30. The prevailing number of rib-bearing vertebrae is 12 or 13, any variation being generally in excess of these numbers. The sacral region offers more difficulties of definition. Sacral Taking the human &quot; os sacrum &quot; for a guide for comparison, vertebrae, it is generally defined as consisting of those vertebras between the lumbar and caudal regions which are ankylosed together to form a single bone. It happens, however, that the number of such vertebrae varies in different individuals of the same or nearly allied species, especially as age advances, when a certain number of the tail vertebrae generally become incorporated with the true sacrum. Other suggested -tests, as those vertebrae which have a distinct additional (pleurapophysial) centre of ossification between the body and the ilium, those to which the ilium is directly articulated, or those in front of the insertion of the ischio- sacral ligaments, being equally unsatisfactory or unpractical, the old one of ankylosis, as it is found to prevail in the average condition of adults in each species, is used in the enumeration of the vertebrae in the following pages. The Cetacea, having no iliac bones, have no part of the vertebral column modified into a sacrum. The caudal vertebrae are those placed behind the sacrum, Caudal and terminating the vertebral column. They vary in vertebrae, number greatly, being reduced to 5, 4, or even 3, in a most rudimentary condition, in Man and in some Apes and Bats, and being numerous and powerfully developed, with strong and complex processes, in many mammals, especi ally among the Edentata, Cetacea, and Marsupialia. The