SKELETON (Gr. skeletos, dry) is the term applied in anatomy to designate the hard parts or frame-work of animals. In the invertebrate animals the skeleton, except in the case of certain corals, is tegumentary or dermal, forming the outer hard and protective cov ering, as in the eekinodermata, mollasea, and crustacea; and like the epidermis and its appendages, is non-vascular, and can only he increased by additions to its edges. This hard insensible covering serves to protect the animal from hurtful external influences, and to afford fixed points of attachment to the muscles which move the body and limbs; the muscles, however, always lying interior to the skeleton, and not clothing it as we see in the vertebrata. We scarcely ever observe amongst the invertebrate that the skeleton bears any definite relation to the nervous system, which is merely protected by it to the same extent as the other soft tissues. Moreover, in none of these animals are the hard parts composed of true bone.
In the vertebrate animals, although we find occasional cases of bone being deposited in various parts of the body, its most constant position is around the central masses of the nervous and vascular systems, with rays extending thence into the middle of the chief muscular masses, forming the bases of time limbs. " Portions of bone are also developed, to protect and otherwise subserve the organs of the senses, and in some species are found incasing mucus-ducts, and buried in the substance of certain viscera— as, c.g , time heart in the bullock and some other large quadrupeds. Strong mem branes, called 'aponeurotic,' and certain leaders or tendons, become bony in some animals—as, e.g., the tentorium ' in the cat, the temporal fascia in the turtle, the leaders in the leg-muscles iu the turkey, the nuchal ligament in the mole, and certain tendons in the abdominal rnweles of the kangaroo, which, so ossified, are called the mar supial bones."—Owete s •tructure of the Skeleton, p. 163. In some animals (e.g., tho sturgeon, the crocodile. the armadillo), bony matter accumulates upon or near to the sur face of the body, rendering the skin in some cases absolutely ball-proof.
In order to give a clear conception of the osseous system, prof. Owen classifies its various parts according to their prevalent position. The superficial or skin bones con stitute the " dermo-skeleton " (Gr. derina skin); the deep-seated bones, in relation to the nervous axis and locomotion, form the " neuro-skeleton" (Gr. neuron, nerve); the bones connected with the sense-organs and viscera form the " splanchno skeleton" (Gr. spin no't
non, a viscus or inward pot); while those developed in tendons, ligaments, and aponcu roses are termed the " selero-skeleton" (Gr. skleros. hard). In the arrangement of the various parts of the derma, splanchno, and sclera skeletons no definite plan or law can be detected. The definite end or purpose gained by the position of the bony plates, cases, or rods, belonging to these skeletons is usually easily seen to be connected with the habits and well-being of the animals in which they occur, hut the parts cannot be referred to one general type, as in the case of the neuro-skeleton. We will follow prof. Owen in taking the sturgeon and armadillo as examples of a dermo-skeleton, and shall condense the remarks which he makes on their outer covering. The head of the sturgeon is defended by a case of superficial bony plates, and the body by five longitudinal rows of similar plates, one extending along the mid-line of the back, 011e along each side of the body, and two along the belly, between the ventral and pectoral fins. These fishes habitually swim low and grovel along the bottom, turning up the mud and sand with their pig-like snout, and feeding on the decomposing organic substances carried down by strong and rapid currents. The heavy dermal osseous plates. regularly arranged in orderly rows along the middle and sides of the body, art as well-arranged ballast. The protection which their plate-a•mor affords them against the logs and stones hurried along their feeding-grounds, renders needless the ossification of the immediate case of the brain and spinal marrow, and consequently all the parts of the neuro-skeleton remain in the flexible, elastic, gristly state common to all the so-called cartilaginous fishes; the weight of the dermo-skeleton requiring that the neuro-skeleton shall be as light as pos sible, consistently with the defensive and sustaining functions which it is called to per form. The coat of mail in which the ganoid fishes of an early period were clothed was probably subservient. to the same ends as the dermal plates of the sturgeon; and in most of these fishes, as in the sturgeon, the dermal bones are coated externally with a very hard material resembling enamel, In these extinct fishes, the plates are more close-set than in the sturgeon, overlapping each other, and being fastened together like tiles by a peg of one entering a socket in the next, and conversely.