SKELETON OF OSSEOUS FISUES.—In osseous Fishes, we shall regard the skeleton as being composed of the head, of the respiratory appa ratus, of the trunk, comprising the body and tail, and of the limbs, viz , the pectoral and ventral fins. The vertical fins, viz, those of the back, anus, and tail, may be regarded as form ing part of the trunk.
The head having more moveable appendages than that of Quadrupeds must be divided into a greater number of regions. We may distinguish in it the cmnium, the jaws, the bones placed un der the cranium behind the jaws, serving for their suspension and motions; the opercular bones, forming. flappers, which open and shut the openings of the branchim ; the bones surround ing the nostril, which are nearly external, as also are those around the eye or the temple, or which cover a part of the cheek.
The respiratory apparatus comprises the os hyoides and its appendages, that is to say, the branchiostegous rays and the arches supporting the branchia, as also the different pieces at tached to these arches, and which altogether perform the functions of larynx and of trachea; lastly, the bones placed at the entrance to the pharynx, forrning in some measure a second pair of jaws.
The trunk is composed of the vertebra of the back and tail (for we can hardly say there is a neck, neither is there any sacruna,) of the ribs, of the bones called interspinous, which support the dorsal and anal fins ; also the mys of these fins, as well as of the tail. These mys, whether they have branches or articulations, or are simply spinous, are always divisible into two lateral halves. There is mrely a sternum, pro. perly so called, in Fishes; and when it exists, it is formed of pieces which are almost external, and which unite the lower extremities of the ribs.
The anterior extremity or pectoral fin com prehends the shoulder, which is an osseous semicircle composed of many bones, suspended at the upper part to the cranium or spine, and uniting inferiorly with its fellow of the opposite side. We may bare find bones analogous to the two pieces of the scapula of Reptiles, to the humerus and to the bones of the forearm ; there is even generally a process composed of two pieces protruding backwards, in which we might seek to see the coracoid bones and even the clavicle.
The two bones comparable to the radius and ulna carry at their edge a row of ossicula, which appear to represent those of the carpus, and which support the rays of the pectoral fin, Nvith the exception of the first, which articu lates at once with the radial bone.
The posterior extremity is much more va riable in position than among Mamnialia; its external or moveable portion, called the ventral fin, emerges sometimes before, sometimes behind, and sometimes immediately beneath the an terior extremity. The pelvis is composed of four bones, the largest and most constant of which, being always in front of the anus and genital orifices, may be considered as a sort of pubis, and these carry on a part of their posterior edge the rays of the ventral fin, without interme diate bones which can correspond either to femur, tibia, fibula, or tarsus. The rays of the pectoml and ventral fins, as of those of the single ones, are divisible longitudinally into two portions.
Vertebral column.—The vertebra of a Fish are at once recognisable by the deep conical cavities which form the articulating surfaces whereby they are connected together, so that a double hollow cone always occupies the in terval between two vertebra, which in the living state is filled up by a soft membranous and gelatinous substance, vvhich passes from one intervertebral cavity into another through holes which generally perforate the centres of the bodies of the vertebra.
In Fishes, as in all other animals, each ver tebra presents superiorly a ring for the passage of the spinal medulla bounded by the superior spinal lamina (neurapophyses), which is gene mlly surmounted by a long spinous process, ( fig. 493, 4,) at the base of which are situated both upon the anterior and posterior aspect little eminences that correspond to the articulating processes of other Vertebrata; but most gene rally these processes only touch or slightly overlap those of the neighbouring vertebra without their being connected together by articulating facets. Sometimes, indeed, they exist on one side of the vertebm and not on the other, so that they have no corres pondents wherewith to articulate. The an nular part of the first vertebra is frequently separated from the body during the whole life time of the Fish, but in the other vertebrte no such separation is visible.