CHAPTER VIII.

THE VERTEBRAL COLUMN OF FISHES.

FROM these creatures-partly reptiles, partly fishes-we come to the class of vertebrate animals to which the term fishes is with propriety restricted. We have already given a general sketch of the leading characteristics of this class, and, in tracing the modifications of the anterior extremities, we were led to a consideration of the pectoral, and also the ventral fins of these aquatic creatures; and, although we abstained from minutiæ, we ventured to show some of the most remarkable modifications to which these analogues of the limbs of the higher animals are subject. We described the pectoral and ventral fins as combining with the tail to fit these scaly tenants of the waters for easy motion in their native element; and we have now to show how the spine harmonizes with the structure

of those fin-limbs, and indeed with every other part of their organization, being, in fact, what it ought to be in animals so expressly formed for the medium they tenant, and the part assigned them in creation.

Let us take one of the fishes with a truly osseous skeleton, say the perch, and investigate the details of the spine. From the head to the caudal fin, a column of vertebral bones is carried nearly in a straight line, or in a line more or less arched; and to these bones and their mode of articulation, the vigorous actions and the flexibility of the tail are owing. In the mammalia, the vertebræ are divided into cervical, dorsal, lumbar, and so on; but in fishes, no such division is admissible. Fishes have no true neck, and no chest; they breathe by means of gills on each side of the head at its junction with the body, and are consequently destitute of those voluminous lungs, which, with the heart, are contained in the chest of quadrupeds. Yet, there is a division in the vertebræ of fishes, for the more anterior are furnished with ribs,* varying in development; in the perch they are rather slender,

* In some fishes, the ribs are mere rudiments, and in others, as the skate, they are wanting.

but are large and strong in the carp; to these rib-bearing vertebræ the name of abdominal has been given, while the rest are termed caudal. The bodies of the vertebræ are short, and consequently the spine is made up of numerous pieces, a circumstance which, conjoined with their mode of articulation, contributes to its elasticity. If we look at a detached vertebra, we shall find that at each end it is hollowed out into a sort of cup-like cavity, or a conical depression. In their natural situation, the edges of these cups are united by a very elastic ligament, and the hollow part, or double cone, (formed by the two cups joined rim to rim,) is filled up with a highly elastic glutinous substance, which allows not so much a free play of one vertebra on another, as a general mobility; the extent of motion possessed by a single vertebra is multiplied in the whole series forming the spine, the elastic mobility of which, thus obtained, is very considerable. When the vertebræ cease to be put into motion, or bent by the action of the muscles, the column resumes its linear direction, the ligaments that unite the edges of the cups, rendering it straight by an involuntary contraction. It is evident, therefore, that the

vertebral column cannot be bent at an abrupt angle, and whoever will watch the actions of a fish in the water will see, that whatever degree of flexure the caudal portion may display, the curve is regular and graceful.

It is only laterally in such fishes as the perch, carp, salmon, and others, that the flexure of the spine occurs; the muscles being so arranged as to act accordingly. Yet it may be observed, that the processes of the vertebræ, the bodies and union of which we have described, forbid flexure in any other direction; thus the vertebræ, and the muscles that govern them, are in just harmony. Each vertebra gives off a large upper spinous process, arising from a double origin, so as to form an arch at its base. The multiplication of these arches forms a canal for the lodgment of the spinal cord, which is thus protected as it runs along the spine. Similar processes, called the inferior spinous processes, proceed from the under part of the bodies of the caudal vertebræ, and through the canal at their base the aorta is transmitted in its passage along the spine.

The number of the vertebræ varies considerably in different fishes. In some we count only from thirty to forty; but the spine of the

shark is composed of upwards of two hundred.

When we speak of the ribs of fishes, it must not be supposed that they subserve the same purpose as the ribs in quadrupeds or birds; they are imbedded in the muscles of the sides, to which they give attachment, and, in many fishes, are furnished with numerous slender appendages, buried also among the muscular fibres. In the herring, these are very abundant. We shall not here notice the interspinous bones running down the ridge of the back, nor those forming the fin-rays, which are either hard and spinous, as in the perch, or soft and cartilaginous, (with the exception, sometimes, of the first dorsal and pectoral rays,) as in the salmon, trout, carp, etc. These bones must be regarded as appendages, which are infinitely varied, and which do not necessarily enter into the composition of the spinal column even as adjuncts.

We have described the structure of the ver

tebral column as we find it in the osseous fishes-fishes in which the skeleton is the most completely ossified-and altogether the most elaborate. There is, however, an extensive group of fishes in which the skeleton is never ossified, but remains permanently in a state of