Skull.—The osseous framework of the head appended to the anterior termination of the spinal column is by far the most complex part of the skeleton, being composed of very nume rous bones connected together by suture or otherwise, but differing marvellously in their character and entering into the formation of very numerous and diversified sets of organs, which have in, reality no alliance with each other except that of mere juxta-position.
One compartment in Man, exceeding in size all the rest put together, but in the lower Ver tebrata forming but a very small part of the whole, is obviously merely a continuation of the vertebral canal lodging the most anterior ganglia of the cerebro-spinal axis, which it arches over and defends, at the same time affording passage to the nerves that emanate therefrom, being essentially itself composed of vertebrie, although, in consequence of the preponderating size of the brain over the spinal ganglia behind, considerable distortion is re quired, a distortion which in human beings is necessarily carried to such an extent that the normal construction of this part of the skeleton is in man alniost wholly indistinguishable. As the vertebral column forms the centre and sup port of the trunk and limbs, so does the cranial portion of the skull sustain various additional apparatus, which may be enumerated as follows.
1. The auditory apparatus most frequently en closed in aspecialbone, the petroas, and interca lated among the proper bones of the cranium.
2. The temporal apparatus, which in man is confused into a single irregular mass that forms part of what the human osteologist calls the temporal bone, but which in the lower Ver tebrata, such as the Reptilia, consists of several important pieces, which being withdrawn from the composition of the cranial box are employed for the articulation of the lower jaw, and more over in the osseous Fishes sustain the bones of the gill-covers. ( Nos. 12, 13, 23, 26, 27.*) 3. The pterygo-palatine apparatus repre sented in the human skeleton by the intei nal pterygoicl processes of the (so-called) sphenoid and the ossa palati. These form the framework of the fauces. ( Nos. 25, 22.) 4. The olfactory apparatus, into the compo sition of which enter the cethinoid, over which the nerve of smell is more particularly dis tributed, together with the nasal, the superior maxillary, the roomer, the iitferior turbinated bones, and others more remotely connected with the formation of the cavity of the nose. (Nos. 3, 20, 16, 18, d.) 5. The orbito-lachryinal apparatus, or the bones which assist in forming the orbital cavity and lachrymal passages.
6. The superior maxilla formed of the max illary and intermaxillary bones. ( Nos. 18,17 .) 7. The inferior maxilla, which in the lower animals consists of several pieces, to be more fully noticed hereafter.
Before proceeding to describe the individual bones that enter into the composition of the cranial portion of the skull, in order to lay before the reader the comparative structure of that important portion of the skeleton, it will be proper to examine how far it is entitled to be looked upon as we have already stated it to be, as forming a continuation of the spinal column, and if so, to define the vertebrx of which it consists. In the human cranium indeed this would be no easy task, partly in consequence of the extreme exaggeration of every element composing it, and partly from the manner in which some bones, distinct in the lower animals, are here consolidated into single masses ; more over in consequence of the prodigious develope rnent of the cerebral hemispheres every part is dis torted and pushed aside as it were out of its pro per situation relative to the neighbouring bones.
In the cranium of the Reptile, however, and even of the less intelligent Mammalia, these difficulties are to a great extent done away with, and the vertebral form is preserved, while, in addition, the elements composing them fre quently remain permanently disunited.
The first cranial vertebra (commencing from behind) is the occipital, and this can present no difficulty. In Fishes, indeed, and in many Reptiles, the occipital bone, of which this ver tebra is entirely made up, has not only the exact shape of one of the spinal bones, but the elements composing it remaining often perma nently disunited, they are most easily and satisfactorily identified. Inferiorly there is the body (or basilar bone, 5) connected with the body of the first spinal vertebra in the same manner as the corresponding portion of the other vertebrm are connected with each other. On each side the neurapophyses (or extra-occi pital bones, 10) arching over the commence ment of the spinal cord, and lastly, the neuro spine (or supra-occipital bone, 8) occupying its normal situation, and in many of the lower Vertebrata forming a real spinous process, although in the human subject, owing to the prodigious size of the hinder part of the ence phalon, it is enormously spread out in propor tion to the dimensions of the parts it protects.