OCR Text |
Show 1880.] VARIATION IN E Q U U S ASINUS. 3 generally short, although we have seen it long and connected with the first rib below the upper head of scalenus by an elastic band; and in a specimen now before us the transverse processes are asymmetrical, several inches in length, prolonged by well-marked elastic bands to cartilaginous nodules appended to the "prosternum in front of the first sterno-costal articulations. Dorsal and Lumbar.-The dorsal intervertebral gaps are sometimes truly intervertebral; often each is wholly pierced through the pedicle of one vertebra. The degree of separation of the costo-trans-verse articulation from that for the head of the rib varies much in the posterior dorsal vertebrae of different individuals. The last ribs also vary much in length, size, and degree of curvature. The presence of a so-called "floating rib," making the series nineteen in number, is frequently observable. This "floating rib" is generally present only on one side. It is not a vertebral rib, but a sternal rib ; for it is appended to the extremity of one or two lumbar transverse processes, the homotypes of the vertebral costse of the dorsal region. Often, when not represented by bone, it occurs as a portion of cartilage or a band of white fibrous tissue, embedded in the abdominal muscles in such a manner as to remind us of the homotypical concordance of the intercostals and the abdominal muscles. In the sacral region it is often difficult to define the extent of the "false sacrum" backwards ; for coccygeal vertebrae become appended by ankylosis, increasing apparently the ordinary number of five sacral bones. The last lumbar, too, sometimes assists in the support of os innominatum; and in other respects the "true sacral bones" are not always the same. Coccygeal.-Vary in number, especially with age. The peculiar modifications which we have noted in the seventh cervical, the uncertainty observable in the anterior and posterior parts of the lumbar and sacral regions, and the variation in the number of ribs prominently bring to our mind the question " H ow is the vertebral column becoming modified in the present day?" and also, " Will an examination of these points throw any light on the remarkable preponderance of dorso-lumbar vertebrae and costae in Perissodactyla ungulata ? " SJcull.-Exhibits many minor variations, most of them probably sexual or due to age. Limbs, Fore.-Scapula : differences in figure, thickness etc. Often, instead of the gradual disappearance of the spine inferiorly, it terminates in a slightly prolonged process, a rudimentary acromion similar to that seen in the Ox, but smaller. Medullary foramen varies in position ; also glenoid cavity rounded or oval. Humerus: synovial fossettes vary in size and form, as do those in upper part of radius and ulna. The ulna generally extends downwards only two-thirds of the length of the radius; but in the foetus it is much longer in proportion, and in the adult we occasionally find it passing downwards to the supero-external part of the knee to articulate with os cuneiforme. If we examine the inferior extremity of the radius of the fcetal colt, we may note that at its external part is 1* |