Cranial and facial bones of the Perch: basilar view.

Fig. 312. Cranial and facial bones of the Perch: basilar view. (After Cuvier).

(1714). Pterygo-Palatine And Temporal System Of Bones

Upon each side of the head is situated a somewhat complex apparatus, connected on the one hand with the articulation of the lower jaw, and on the other with the opercula, or gill-covers. These bones are seven in number on each side.

(1715). The palatines (22) are easily recognizable, forming part of the roof of the mouth, and generally armed with teeth.

(1716). Two bones are connected with the posterior edge of each palate bone: one, situated externally, becomes, in Reptiles, a very important element; it is called the transverse bone (21); the second (25) is named the internal pterygoid.

(1717). The other pieces belonging to this part of the skeleton are not a little interesting on account of their remarkable arrangement; and perhaps the anatomical student will be somewhat startled at the position which some of them occupy. In the first place, the squamous portions of the temporal, instead of entering into the formation of the cranium, are here slightly displaced, and, although still called the temporal bones (23), are articulated by a hinge-joint with the posterior frontal and mastoid bones, and thus form a moveable basis to which the opercular apparatus is attached.

(1718). Connected with the temporal we have the broad and flat piece, 27, which is the tympanic bone; and to these the pieces forming the opercula are appended.

(1719). Lastly, supporting the lower jaw we find the jugal bones; and connecting these with the rest of the temporal apparatus are two small ossicles (31), which complete this portion of the skeleton.

(1720). The seven bones above enumerated are almost immoveably connected with each other by the interposition of cartilage between their edges - a mode of articulation distinguished by the name of synchondrosis', but the whole apparatus moves readily upon two hinges, one formed by the articulation of the palate bone with the maxillary and vomer, and the other by the joint which unites the temporal bone to the posterior frontal. This movement, by opening the gill-covers, enlarges the cavity of the mouth when the fish wishes to take in the water necessary for respiration, or else, by acting in a contrary direction, again expels it.

(1721). Opercular Bones

The great flap, which in osseous fishes closes the gill-openings externally, is composed of four pieces, to which the following names have been given. The praioperculum (30) is attached to the posterior edge or angle of the palato-temporal apparatus last described, and its borders often present spines and indentations, which, being visible externally, are of much importance to the ichthyologist, as they afford a good character of distinction between allied genera. The second piece (28), which from its size is called par excellence the operculum, together with the subopercidum (32) and the interoperculum (33), form a flap which covers the gill-opening like a great valve, opening and shutting continually to give exit to the water used in respiration.

(1722). Lower Jaw

The lower jaw of Eishes consists of two lateral halves united by a symphysis in the mesial line, each branch being articulated with the jugal bone of its corresponding side. Each division is separable by maceration into four or even five pieces: viz. the dental (34), which supports the teeth; the articular (35), bearing the articulating facet; the angular (36), forming the angle of the jaw; and a fourth, placed upon the inner surface of the articular, called the opercular, because it corresponds with a bone met with in the lower jaw of reptiles, to which the same name has been applied. The fifth, when present, is very small and unimportant.

(1723). Os Hyoides And Branchiostegous Rays

The os hyoides of a fish is situated as in other vertebrate animals; it is composed of two branches, each made up of several pieces (fig. 313, 37, 38, 39, 40), and is always suspended from the temporal by means of two small ossicles (59), which, as they represent the styloid process of Man, are called the styloid bones.

Os hyoides and branchial bones of the Perch. (After Cuvier.)

Fig. 313. Os hyoides and branchial bones of the Perch. (After Cuvier).

(1724). Between the two branches of the os hyoides is placed a single central piece (42), which becomes of great importance in Reptiles and Birds; and upon this is the bone which supports the tongue, or the lingual bone (41).

(1725). The great fissure that exists on each side between the head and shoulder of an osseous fish, wherein the gills are situated, is not closed merely by the opercular bones, but likewise by a broad membranous expansion called the branchiostegous membrane, which is adherent to the os hyoides, and assists in forming the great valve of the operculum. This membrane is supported by a series of slender bones derived from the external margin of each branch of the os hyoides, and these are named, from their office, the branchiostegous rays (43).

(1726). Branchial Apparatus

Fishes breathe by taking water into their mouths, and forcing it out again through the apertures situated upon each side of the neck; it is thus made to pass between their gills, which form a series of pectiniform vascular fringes supported upon a system of bones called the branchial arches. The branchial arches, which are generally four in number on each side, are attached by one extremity to an intermediate chain of bones (53, 54, 55) situated in the mesial line behind the os hyoides, whilst by their opposite extremity they are connected by ligaments to the under surface of the cranium.