OSSEOUS SYSTEM), but are so intimately lated to the real bones both in structure and office, that they must be described in this place as being essentially connected with the bony framework of the body.
Every one of the vertical rays entering into the composition of these fins consists of two portions, an interspinous bone, which is im bedded in the flesh of the fish between the great lateral muscles, and serves as a basis to which the ray is attached, and the ray itself, which is visible externally and generally assists in supporting the membrane of the fin.
Interspinous bones.—The interspinous bones form a series reaching along the back to an ex tent proportioned to the length of the dorsal fin, and in a similar manner are appended be neath the post-abdominal region of the ventral surface coextensively with the anal fin, which they resemble. Each interspinous bone (fig. 493, 3, 3, 6, 6) resembles in its shape the blade of a dagger plunged into the flesh, while its head cor responding with the handle of the dagger remains on a level with the skin to give attachment to the base of the ray. This portion of the inter spinous bone has an apophysis conjoined with it by suture, which in many instances is pro longed into a point that is connected to the articulation of the next ray of the fin. The interspinous bones are generally so disposed that their points penetmte between the vinous processes of the vertebrw, each being attached to these processes by a ligamentous membrane; but there are some Fishes, as the Pleuronectide (fig. 493), and (as regards the composition of the anal fin) the Siluri, in which there are two interspinous bones to each spinous process, and in other cases the relations between the two be come quite lost, as, for example, where three or even four spinous processes are interposed be tween some of the vertebral spines, a fact which in itself is sufficient to disprove the hypothesis of Geoffroy, extensively promulgated in this coun try by the writings of Professor Gmnt, namely, that the interspinous bones of Fishes are dis memberments of the spinous processes, one half of the latter becoming displaced and forming the interspinous bone as well as the ray which it supports.
On this point Cuvier remarks that in many genera, such as Murxna, Ophicephalus, and Gymnotus, the inferior interspinous bones are separated from the vertebrx by the cavity of the abdomen, which is prolonged to a consi derable distance beyond the commencement of the anal fin ; whilst in other cases, as in the Pleuronectidx, there are interspinous bones even upon the cranium (fig.493,1). These cir cumstances, joined to the fact that in those por tions of the back or of the tail which have no fins attached to them there are generally no interspi nous p rocesses al though there are vertebral spines, make it impossible to regard the bones in, question as being derivations from the vertebral column.
Rays of the vertiealfins.—Each fin-ray (fig. 493, 2,7) is connected with its corresponding, interspinous bone by a ginglymoid articulation. The rays are of two kinds : spinousrays,such as are met with in acanthopterygious Fishes, and branched or soft rays such as are found in the Malacopterygii. They are all divided by a longitudinal raphe, or suture, into two lateral halves, so that each appears to be formed of two rays conjoined,—a circumstance .which forms an additional argument against these parts of the skeleton being dismemberments of the vertebrx.
The rays of the caudal fin are always soft and articulated ; but in many Fishes some of those at its root, both above and below, are gradually diminished in size until nothing is left of them but the hard part forming the base.
Skeletons of Chondropterygii.—In the true cartilaginous Fishes, such as the Sharks and Rays, the bones are always destitute of those osseous fibres which give hardness to the skele ton in the preceding races possessing a true bony skeleton. Their interior remains perma nently soft and cartilaginous, while their ex ternal surface is strengthened by becoming en crusted with a layer of granular-looking calca reous matter.