CRANIUM (in anatomy) Gr. /egavcoy; Fr. Crane ; Germ. Hirnschadel ; Ital. Cranio.
The cranium is the protective investment of the brain, on which it is moulded, and the form of which, in warm-blooded animals, it represents. It also incloses and protects the organ of hearing.
In cold-blooded animals there is not this adjustment of the surfaces of the brain and its case; but, although in them the parietes of the cranium are expanded beyond the limits of the brain,'1' the principle of formation is neverthe less the same; and a glance at the several classes of vertebrated animals will demonstrate that security for the brain is the grand aim of the contrivance, and that the modification it sustains in the case of Fishes and Reptiles is for the purpose of carrying into effect some additional design.
Considering the cranium as a capsule for the brain, its form is necessarily determined by the extent to which that organ is developed in the several classes of animals ; while, at the same time, the nature of its organization is in harmo nious correspondence with their habits and with the external circumstances by which' they are surrounded. By pursuing this inquiry from the lowest to the highest animals, it will be per ceived that, as respects both form and struc ture, additions are made in proportion as the endowments are of a more and more exalted character • and further, that these successive changes of structure are the changes vvhich the human skull itself experiences in its progress from a feetal to an adult condition.
The rudimentary part of the most elaborate cranium is a sac consisting of two membranes and an intervening gelatinous fluid ; in the next step of the formative process, this gelati nous fluid gives place to cartilage. A deposi tion of earthy matter in this cartilaginous nidus gives it firmness, but breaks up the sac into isolated ununited patches. These., isolated patches coalesce in definite numbers, and thus establish a secondary and less numerous divi sion of ununited parts; these, in their turn, approacb and combine with each other, form ing a solid case of bone; and lastly, this solid case resolves itself into two tables of different structure, and a still further differing connect ing medium. In each and all of these states
through which the crania of the Mammalia pass there is presented to us a type of the skull in some lower animal.
In Fishes the cranium is little more than a tubular continuation of the spine through the head to contain a similar prolongation of the medulla spinalis. These, however, are not in contact. A mass of reticulated membrane, holding in its cells a gelatinous fluid, forms the real superior investment of the brain ; while the superjacent parietes are designed to aflbrd an extensive origin to the muscles of the body; and as these muscles increase, so does the sur face of their attachment. For this purpose it is. that the ossific deposits remain ununited, that, by being simply in juxtaposition, or at most over lapping each other, they may unfold them selves, and thereby admit of the bead being at all times in proportion to the rest of the body.
In Reptiles the skull is still further deve loped. It is charged more with earthy than with animal matter; and this being loosely distributed, tough spongy bones are the result. The tardiness of their circulation does not favour the combination of the individual por tions, and the bones are therefore for the most part loose, although some of them unite by a species of anchylosis in the direction in which defence is required.
In Birds the character both of form and. structure is greatly changed ; light, fragile, and compact, it is (by reason of the high state of vitality which prevails) so rapidly and com pletely ossified over its entire surface as to afford no evidence, or but a very slight one, of its original subdivision. In conformity with the development of the brain, it extends itself backwards, to each side, and upwards as well as forwards, thus constituting a considerable portion of the entire head.