This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2167). The occipital bone consists primarily of the same pieces as in the Reptile; but in the Mammifer these are at an early period consolidated into one mass, situated at the back of the cranium. Its basilar portion (5) articulates with the atlas by two condyles; while the lateral wings (10) and the superior arch (8) surround the foramen magnum, and protect the cerebellic regions of the encephalon.
(2168). The sphenoid (6), although composed of fewer separate pieces than in the Reptilia, and even regarded by the human anatomist as a single bone, is still distinctly divisible, especially in young animals, into two very important portions - one anterior, and the other posterior - each, as we shall soon see, forming the body of a distinct cranial vertebra. The posterior half (6) consists of the body, including the posterior clinoid processes, and of the greater alse and pterygoid processes (fig. 381, 25.) The anterior half is formed by the anterior clinoid processes and alae minores (fig. 381, 11.) These two halves may therefore be called, respectively, the anterior and posterior sphenoids.
(2169). Lastly, we have the temporal bone, exhibiting but one piece, although made up of all the parts which in the Reptile were so obviously distinct elements. The petrous portion, wedged into the base of the cranium, still encloses the internal car. The tympanic element (fig. 380, a) supports the membrana tympani. The mastoid process (fig. 381, 12) is the homologue of the mastoid bone of the Crocodile; and, lastly, the squamous element, with which the lower jaw is articulated (fig. 380, 23), in the Reptilia was visibly a distinct bone. Even to these may be added the zygomatic process, which Professor Owen regards as an independent elemental part.
(2170). Reviewing, therefore, all that has been said relative to the composition of the skull in the different classes of Vertebrata, the following deductions may be arrived at * -
1. That as we advance from lower to higher forms, the proportionate size of the cranium relative to that of the face becomes greater.
2. That the number of bones met with upon the inferior and lateral aspects of the head gradually diminishes: for in Mammalia the pterygoid and tympanic bones, which even in Birds are separate pieces, become very generally confounded with the sphenoid and the temporal: and also the petrous and squamous portions of the temporal become blended together.
3. The number of bones normally entering into the composition of the cranium of adult Mammalia varies considerably. When most numerous, there are twenty-eight - eleven in the cranium, and seventeen in the face. In this case the cranial bones are the following: - one occipital, one sphenoid, the two squamous portions of the temporal, the two tympano-petrous portions of the temporal, the two parietal, the two frontal, and the ethmoid.
(2171). The bones of the face are: two superior maxillary, two intermaxillary, two nasal, two lacrymal, the vomer, two inferior turbinated bones, two palate bones, two jugal bones, and, lastly, the two halves of the lower jaw.
* Meckel, Traite Generale d'Anatomie Comparee, torn. iii. 2de part,, p. 195.
(2172). It is true that some slight exceptions occur: thus, forexample, in the Cetacea the pterygoid bones remain detached; in the Rodentia the occipital is divided into a superior and inferior portion; but in the latter, the two frontal and the two parietal become consolidated into one bone.
(2173). In Man the bones of the cranium become much less numerous, inasmuch as all the elements of the occipital, of the temporal, of the frontal, the intermaxillary, and the maxillary, composing the upper jaw, and the two halves of the lower jaw, respectively coalesce; and the skull consists of but one-and-twenty bones, - seven in the cranium, and fourteen in the face.
(2174). Even this number is not the smallest; for in some Monkeys the nasal bones unite and become consolidated into one piece.
(2175). Having thus enumerated the different osseous pieces forming the crania of all classes of vertebrate animals, we must next consider them in another point of view, namely, as being continuations of the spinal chain of bones, or real vertebrae modified in form and proportions conformity with the increased volume of the nervous masses they are ined to enclose. "We must premise, however, that it is by no means tention to adopt unreservedly the theoretical opinions of those mental writers who find vertebral elements in the bones of the face, even in the nasal cartilages; still, without overstraining the facts, is easy to demonstrate very satisfactorily that the cranial pieces that immediately enclose the cerebral masses are strictly vertebrae, and present the same essential structure as those of the spinal region.
(2176). That this is the case in the skull of a Reptile, no one, indeed, who examines the subject can hesitate to admit; but even in the Mam-miferous cranium, where, from the enormous proportionate size of the encephalon, the cranium is most distorted, it is not difficult to perceive the relationship.
(2177). The cranial vertebrae are three in number: the occipital, the parietal, and the frontal; these are exhibited in the subjoined diagram, after Cams, representing those of the Sheep.
(2178). The occipital vertebra (fig. 382, a) has for its body the basilar portion; the arches bound the foramen magnum laterally; and above, the spinous process, flattened out and expanded in proportion to the size of those lobes of the brain and cerebellum which it defends, forms the posterior portion of the skull.
(2179). The body of the second or parietal vertebra (b) is the body of the sphenoid - that is, more properly speaking, the posterior sphenoid bone, whose large alae, curving upwards, meet the parietal, and thus an arch is formed of sufficient span to cover the middle lobes of the cerebrum.
(2180). The anterior, or frontal vertebra (c) has for its body the anterior sphenoid (alee minores); its arch being completed by the cavity of the os frontis, which encloses anteriorly the cribriform plate of the ethmoid bone.