16 EVERS'S COMPARATIVE ANATOMY. 



present, consists, as described by Meckel, of a variable number of 

 V-shaped pieces, pointed downwards and overlapping each other. 

 This rudimentary bone is best seen in the herring and the dory. 



Head. — To the researches of Oken, Meckel, Cams, Blainville, 

 &c, we are indebted for the important fact that the cranium, is 

 nothing more than a highly developed portion of the vertebral 

 column. The composition of the cranium in the cartilaginous 

 fishes is very simple; in many, as in the skate, consisting chiefly of 

 one large piece. In the osseous tribes, on the contrary, the com- 

 ponent parts are very numerous — amounting to 80 in the head of 

 the perch. The bodies of the vertebras continue forward in a straight 

 line with the spine along the base of the scull, forming the basilar 

 part of the occipital, the body of the sphenoid, and the ethmoid 

 bones. In the bony fishes the cranium is remarkable for being thin, 

 diaphanous, elastic and having its elements united by squamous 

 sutures, which favours the extension of the period of growth of each 

 part. The cavity of the cranium is occupied chiefly by the cellular 

 tissue of the arachnoid coat; the brain occupying but a small por- 

 tion of the base of the skull. 



The bones of the face present but few peculiarities. The inter- 

 maxillary bone consists of three triangular pieces inserted between 

 the vomer, the palate, the nasal, and the superior maxillary bones. 

 The central piece is generally cartilaginous. In connection with 

 the upper jaw, we have to notice the os quadratum ; this is composed 

 of several pieces, which closely resemble the ascending process of 

 the lower jaw in man. The three portions of which it is mainly 

 composed are articulated, one to the inferior maxilla and palate 

 bone, the second to the temporal bone, while the third consists of a 

 thin round plate, and is called os discoideum. Behind the lower 

 jaw, and connected to the os quadratum, is the operculum. This 

 is a plate of bone usually composed of four pieces, which have been 

 considered analogous to the bones of the tympanum, on account of 

 the absence of this cavity from the ears of fishes ; others have 

 regarded it as a portion of the lower jaw. Be this as it may we find 

 the operculum covering the respiratory apparatus in the same man- 

 ner as the shells of bivalves cover the subjacent gills. 



The os hyoides reaches a high degree of development here, as in 

 all water breathing vertebrata ; it consists of a body or lingual bone, 

 and five pieces on either side of it: it is suspended from the tem- 

 poral bones ; and it is, by its free antero-posterior motions that res- 

 piration is effected in fishes, and in amphibia, as will be shown 

 hereafter. It forms an arch, having the lower jaw above it, and the 

 scapular and coracoid bones below. Its sides support the four pairs 

 of branchial arches, and externally it has attached to it the opercular 

 membrane. 



Extremities. — In fishes fins supply the place of extremities ; the 

 anterior corresponding to the arms, and the posterior to the legs. 

 They are named according to the parts they are attached to, as 

 dorsal, pectoral, ventral, caudal, and anal fins. The anterior or 



