FISHES. 271 



fishes, in which, whilst the rest of the skeleton' is increasing in hard- 

 ness, some part or another is sure to remain cartilaginous, and this 

 is really exemplified in the head of the Pike. 



The Joints of the Bones of Fishes. 



The articulations, or joints, of fishes, exhibit the same varieties 

 as are observed in those of other animals, save only, that those of 

 the arthrodial and ginglymus kind, or whose articulations which allow 

 of determined motions, either according to a single plan or to many, 

 are rarely to be found in the fishes, inamuch, as their limbs are 

 not required to perform any varied series of motions, (a) It is by 

 means of a ginglymus, for instance, that the lower jaw and the oper- 

 culum are attached to the pterygo-palatine apparatus, and the latter to 

 the cranium. We observe it also in the joints of the rays of the 

 dorsal and anal fins formed with the interosseous bones, and in the 

 joint of the first ray of the pectoral fin with the bone, which in 

 the fish, analogous to the radius. (Jb') In the fishes there are two 

 species of joints for a given extent of motion, of which avc have no 

 instance in any- other class : Ave allude to the rings Avhich are joined 

 to each other like the links of a chain, and which, according to the 

 impulse given them by the fish, become very active or completely 

 fixed ; examples of the one kind and the other, are to be met with in 

 the Silures. The joints, destined for a certain extent of motion, like 

 those of the superior animals, have their ligaments, cartilages, and 

 synovia, (c) 



The articulation of the bodies of the vertebrae is formed by means 

 of a fibro-cartilaginous substance passing through these bodies, and 

 sometimes assumes the form of an elongated cord, as in the sturgeon 

 and lamprey ; it is also by a medium of fibro-cartilage that the pieces 

 of the opercula, those of the branchial apparatus, those of the bones 

 of the shoulder, arms, wrist, bones of the pelvis, and of those which 

 are attached to the bones of the shoulder, become united, the one to 

 the other. 



Chemical Composition of the Bones of Fishes. 

 From the investigations and experiments Avhich my learned col- 

 eague in the Institute. M. Chevreul, has gone through, on my entreatv, 

 it appears that the bones of fishes, as well as those of other vertebrated 

 animals, consist of an organic base (animal matter) penetrated by an 

 earthy substance. 



C^ (a) It may be proper to state, that the Arthrodia or Arthrodial class of joints 

 consists of those in which flat surfaces, or nearly so, are connected. The wrist 

 joint is an example of this class. Tlie ginglymus division of joints consists of those 

 resembling a hinge, as the elbow for instance. 



{b) The radius is the bone which may be felt in a line with the thumb, up to a 

 point opposite the elbow. 



(t) This word is of Greek origin, and means the resemblance to the white of 

 egg. Synovia is an oily albuminous fluid, secreted in membranes between the 

 joints, and operates precisely as grease on the wheels of vehicles.— Eng. Ed. 



