The second or parietal vertebra of the cra nium is slightly more distorted, and its real nature masked, particularly in the higher Ver tebrata, by the interposition of the petro-tem poral bone, which does not nortnally belong to the cranium, between it and the preceding. Its body is the sphenoid bone, represented in the human subject by the posterior part of the sella Tarcica, but which in Reptiles is a dis tinct element of the skull ; its arches are formed by the ale majores of the sphenoid, (likewise separate pieces of the cranium in the lower animals, although in man they are con solidated with the former,) while the spine is converted into the expanded parietal bone or bones spread out over the central regions of the brain.
The anterior cranial vertebra is called the frontal, receding still more from the normal appearance of a vertebra than the parietal, the preponderance of magnitude in the different elements that form it being completely in verted; the body being quite rudimentary, while the enormously developed spinous ments are now converted into frontal bones. In man its body and its arches are represented by the dee minores of the sphenoid or ingrassial bones, and the os frontis constitutes its dispro portionately expanded spine. These three vertebr, therefore, are the essential consti tuents of the skull ; and, although in the hu man cranium the most aberrant of any met with in creation, their nature is not at once obvious; it is only necessary for the student to recur to less distorted forms of the head at once to recognise the reality of the resem blance.
Should other proof indeed be wanting, the manner in which all the cerebral nerves make their exit from the cranium would in itself offer a convincing argument. In every °flier part of the cerebro-spinal axis the nerves in variably are given off through passages situated between contiguous vertebrm, which are called, from this circumstance, par excellence, inter- vertebral .foromina ; nay, so sure is this guide, that in those instances where the vertebral pieces are confused, so as to be otherwise un distinguishable from each other, the position and number of these foramina is sufficient to indicate the number of vertebrm of which the part of the skeleton in question originally con sisted, before the pieces composing it became permanently anchylosed. Precisely in the same manner the nerves derived from the encephalon pass out through the interspaces between the occipital and parietal vertebrte, or between the latter and the frontal ; and, although from the great bulk of the encephalic masses and the number of nerves derived therefrom, the passages through which they principally escape have been named foramina lucent, indicating their size and irregularity ; they are not on that account less the repre sentatives of the intervertebral foramina pro perly so called. The mere circumstance of the
channels of some of these nerves being, in the human subject and in other Mammifera, cir cumscribed by rings of bone and thus con verted into distinct foramina, to which special names have been given by the human osteo logist, militates in no degree against the grand fact that it is between the cranial vertebrx they all make their exit.
Having given the above general view of the composition of the osseous skeleton, a more difficult task now remains to be accomplished, viz. to identify and compare with each other the individual bones entering into the com position of the osseous system throughout the different vertebral classes, and thus to analyse the entire fabric. Various and conflicting in deed are the opinions of different writers on this important subject, of whose names and works an ample list will be given in the Biblio graphy affixed to the end of this article ; but to enter into the argumentation of disputed points would of course be impossible in our prescribed limits. Suffice it to say, that the views of the acute and sound-judging Cuvier have been principally adhered to, and where occasion has been found to dissent from his I opinion we have expressed our reasons for so I doing.* Bones (if the cranium.—Frontals (I). These bones in fishes form the roof of the orbit and the anterior portion of the cranial box, having in front and behind them other pairs of bones forming the anterior and posterior boundaries of the orbit which correspond with the ante rior and posterior frontals in Reptiles. In the Frog the whole of the anterior portion of the cranium is made up of a single bone, which entirely surrounds it like a ring or girdle, and represents the two frontal bones of Serpents (fig. 438, 1) united together. In the Siren and Proteus, however, the principal frontals are divided as in other Reptiles. In all Birds and Mammalia these bones become at an early period confused with the anterior and posterior frontals and ultimately with each other, so as to form but one piece, the os ji.ontis of man ; nevertheless, even in the human fcetus, they are separated by a suture which, in the lower Mammalia and also in the human subject, is not unfrequently persistent to alate period oflife.