Imágenes de páginas
PDF
EPUB

CHAPTER V.

ON THE SPINAL COLUMN OF QUADRUPEDS.

With the condition of the limbs the structural modifications of the spinal column are in admirable accordance. The spinal or vertebral column consists of a series of bones (vertebrce)

с

conjoined together, each bone having a body (A), and certain processes, or additional parts,

encircling an orifice or open ring, so that from their apposition a canal is the result, continued

А

B

E

throughout the column, bounded anteriorly by the body of the bones, and posteriorly by the

processes. Of these processes, or projecting additions just described, one is termed the spinous (B), four are called the oblique (c), and two the transverse (D); and the vertebræ themselves are divided into those of the neck, A (cervical), those of the back, B (dorsal), those of the loins, c (lumbar), and those between the hip-bones, D (sacral.) To these follow accessory vertebræ in a rudimentary state, E, called coccygeal, or caudal; they run down the tail of quadrupeds, and are comparatively unimportant, and ever variable, as will appear by referring to the engraving of the spinal column of a lion. It is to the dorsal vertebræ in mammalia that the ribs are articulated. Each rib has its head fitted into a depression, formed conjointly by the bodies of two vertebræ, and is further united by a sort of tubercle to the transverse process of the lowermost of the two vertebræ to which its head is attached, To this mode of union among mammalia there are exceptions. In the whole tribe, for instance, the anterior ribs are each attached only to one vertebra by the head, and to the transverse process of the next by the tubercle, while the posterior ribs are attached exclusively to the transverse pro

cesses.

In that curious animal, the ornithorhynchus of New Holland, the ribs are attached solely to the bodies of the vertebræ.

The use of the spine is to give firmness to the general frame, yet consistent with ease and grace, and to afford a safe canal for the continuation of the spinal cord from the brain, which, as it passes, distributes through certain openings between the vertebræ nerves to the body and limbs. The mode of union in the mammalia between the separate bones composing the vertebral column is such as to endow the whole with considerable flexibility, while, at the same time, that the spinal cord may not be compressed, the individual mobility of each is very limited. It must be observed, then, that the bodies of the vertebræ are not in close contact, for, in this case, the column would be rigid, but there is interposed between them a substance of considerable thickness-a soft cartilaginous consistence, and highly elastic, to which the flexibility of the spine is owing. It is, in fact, from the interposition of these elastic cushions between the bodies of the vertebræ, (which are united together by the oblique processes, that the flexibility of the spine is owing, so that the whole can be bent

E

forwards, backwards, or laterally to a greater or less degree, without an abrupt angle occuring in any part, to the injurious pressure of the spinal cord. The flexure is always formed by the concurrence of many vertebræ, and not of two, and the arch formed is gradual. Thus the hedgehog or the dormouse may roll itself up into a ball, without disturbing the great cord of nervous communication between the trunk and the brain. In man, who stands erect, the head is nearly balanced upon the spine; and' thus he gazes around him, “monarch of all” he surveys ; but his spine is not a straight column like a ruler, for then, though erect, his form would be destitute of grace, and his movements would be stiff and ungainly. It presents a series of graceful curves, that convert it into a perfect spring. The neck gently arches forwards, and thence between the shoulders takes a slight curve in the opposite direction, and again arches forward along the region of the loins. The vertebræ of the loins are the largest, those of the neck the smallest, and

seven in number; and as the head is balanced, or nearly so, on the spine, requiring no strong muscles to sustain its weight, the processes of

« AnteriorContinuar »