This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1687). There are only two kinds of vertebrae recognizable in the skeleton of a Fish, viz. the abdominal and the caudal. The abdominal vertebrae support the ribs (for in these animals the ribs do not constitute a thorax, or contain any of the viscera called thoracic in the human body); they extend from the head to the commencement of the tail, and are at once recognizable by the nature of the elements which enter into their composition, each vertebra being provided with a superior arch (fig. 306,b), through which passes the spinal cord, a superior spinous process (c), and two transverse processes (d), to the extremities of which the ribs are generally attached. The caudal vertebrae are composed, as we have already seen, of different elements: the transverse processes either do not exist, or are very feebly developed; but beneath the body an inferior arch is formed, and from this an inferior spinous process, equalling the superior in length, is prolonged in the opposite direction (fig. 311, b).
(1688). As the vertebrae approach the tail, they become somewhat modified in structure to support the caudal fin; their spines become shorter and thicker, the canals formed by their superior and inferior arches smaller or nearly obliterated, and at length the spines become, as it were, soldered to each other and to the interspinous bones hereafter to be noticed; so that they form a broad vertical plate, to the posterior margins of which the rays of the tail-fin are articulated (fig. 311, 70).
(1689). The ribs of Fishes are slender bones, appended either to the extremities of each transverse process of the abdominal vertebrae, or else to the body of the vertebra itself: every rib is connected with but one vertebra, and that only at a single point. They do not, as we have already said, form a thoracic cavity, but enclose the abdomen, and are imbedded among the lateral muscles of the trunk, to which they give support. From each rib arises a long styliform process (73), which, inclining backwards, is likewise plunged among the muscular fasciculi; and in some fishes, such as the Herring and Carp tribes, similar appendages are derived from the bodies of the vertebrae themselves, so that the bones of such fishes appear to be extraordinarily numerous. On the other hand, many tribes have but the rudiments of ribs; and in some, as, for example in the Skate, they are altogether wanting.
Fig. 311. Skeleton of the Perch (Percafiuviaiilis.) (After Cuvier).
(1690). No sternum, properly so called, exists in Fishes; but the extremities of the ribs are sometimes connected with ossified plates belonging to the tegumentary system, which cover the abdomen, and which by some authors have been regarded as a sternal apparatus.
(1691). We have now to request the attention of the reader to certain supplementary organs which are peculiar to the class before us. These consist in sundry appendages to both the superior and inferior spinous processes of the vertebrae, which are generally prolonged into fins situated along the mesial line of the body. These single fins, which must by no means be confounded with the pairs of fins that represent the arms and legs, are very variable in their position, and in many cases are altogether wanting. When fully developed, one of them is situated along the mesial line of the back, and in the Perch (fig. 311) this dorsal Jin is separated into two distinct portions (75); another, denominated the caudal Jin, forms the tail; and a third, likewise situated in the median line, at a short distance behind the anal orifice, is called the anal Jin from that circumstance.
(1692). These fins present two sets of bones - the interspinous bones, which form the basis to which they are affixed, and the fin-rays.
(1693). The interspinous bones (fig. 311, 74) form a series of strong dagger-like bones, deeply implanted in the flesh along the mesial line of the body, between the two great masses of lateral muscles: their points generally penetrate to a little distance between the spinous processes of the vertebrae, to which they are connected by a ligamentous attachment; whilst to their opposite extremity, which may be compared to the hilt of the dagger, the corresponding fin-rays are affixed by a beautiful articulation. There is generally only one interspinous bone affixed to a vertebral spinous process, but in the Flat-fishes (Pleuro-nectidce) there are two; and moreover, in that remarkable family, the inferior spinous process of the first caudal vertebra (which, -as we have already seen, is of enormous size) frequently has not fewer than six or seven interspinous bones appended to its extremity.
(1694). Each interspinous bone consists of two pieces, united by a suture, - one portion representing the blade, the other the handle of the dagger, to which we have compared it.
(1695). The fin-rays of Fishes are of two kinds, being either solid, and apparently composed of one strong piece, like those which support the anterior half of the dorsal fin of the Perch (75), in which case they are called spinous rays, or else they are composed of several slender stems derived from one common root, every one of which is made up of numerous pieces: these, which bear the name of soft rays, are found in the posterior portions both of the dorsal and anal fin of the Perch, and are invariably met with in the tail of all fishes possessed of a caudal fin. This difference in the structure of the fin-rays, trivial as it might appear, is a circumstance to which much importance is attached by ichthyologists, who hence derive the means of separating osseous fishes into two great groups - the Acanihopterygii, or such as possess spinous rays in the composition of their dorsal fin, and the Malacopterygii, in which all the fin-rays are soft. Every fin-ray, whether spinous or soft, is in reality made up of two lateral halves placed side by side: in the soft rays these are easily separable; but in the spinous rays they are firmly united along the median line, so as to represent but one bone.