The "cleft" between the palato-pterygoid (II.) and the mandibular (III.) bar is the mouth. The first true cleft, however, is the one between the mandibular and the anterior hyoid arch (IV.). It becomes contracted into the Eustachian tube, tympanic cavity, and meatus auditorius externus, which would form a canal of communication between the pharynx and the external surface but for the interposition of the delicate membrana tympani.

Some of the changes which take place in the cranium while advancing from youth to maturity have already been noticed; but it will be well, before proceeding to describe the modifications of the Mammalian skull, to mention certain others which take place, to a greater or less degree, in all skulls.

These depend mainly on the fact that the brain, and consequently the cavity which contains it, and also the sense capsules, increase in size in a much smaller ratio than the external parts of the head, especially the jaws and prominences for the attachment of muscles. The disproportionate growth and alteration of form of these parts, concomitant with little or no change in the brain-case, is effected partly by increase in thickness of the bones, but mainly by the expansion of their walls and the development of cells within, which greatly extend the outer surface without adding to the weight of the bone.

In

In the Dog these cells are developed chiefly in the forepart of the frontal bones, constituting the frontal sinuses, and in the presphenoid, constituting the sphenoidal sinuses. Air passes freely into them from the nasal passages. many animals they attain a much larger extent than in the Dog, reaching their maximum in the Elephant (see Fig. 64, p. 205), where the alteration of the external form of skull during growth, without material change in the shape or size

of the cerebral cavity, is strikingly shown. At the same time the alveolar borders of the jaws gradually enlarge to adapt themselves to the increased size of the permanent teeth which they have to support, and the various ridges and tuberosities for the attachment of muscles become more prominent.

During these changes a gradual consolidation takes place in the structure of the skull generally, by the partial or complete union of certain of the bones by synostosis. The union of the different bones generally proceeds in a certain definite order, which, however, varies much in different species. Sometimes it extends so far as to lead to complete obliteration of all the cranial sutures.

CHAPTER X.

THE SKULL IN THE ORDER PRIMATES, CARNIVORA,

INSECTIVORA, CHIROPTERA, AND RODENTIA.

Order PRIMATES. Man.-On comparing a longitudinal and vertical section of a young human skull in which most of the sutures are still distinctly seen (Fig. 53), with that of the Dog, it will be seen to be composed of the same bones, having very nearly the same connections, and yet the whole form is greatly modified. This modification is mainly due to the immense expansion of the upper part of the middle or cerebral fossa of the brain cavity, which not only carries the roof of the cavity a great distance from the basicranial axis, but also forces, as it were, the anterior and posterior walls from the vertical nearly to the horizontal position, so that they are, roughly speaking, in the same line with the short basicranial axis, instead of being perpendicular to it. In addition to this great difference, the facial portion of the skull is deeper from above downwards, and very much shorter from before backwards.

Taking a survey of the human skull in the same order as was done with that of the Dog, we find the craniofacial axis, composed of the basioccipital bone (BO), terminating at the anterior border of the foramen magnum (fm) behind,

FIG. 53. Vertical, longitudinal, median section of a young human skull, with the first dentition,. As in the other sections of a skull figured, the mandible is displaced downwards, so as to show its entire form. PMx premaxilla; MT maxilloturbinal; ET ethmo-turbinal; ME ossified portion of the mesethmoid; Na nasal; cg crista galli of the mesethmoid; OS orbitosphenoid, or lesser wing of the sphenoid; AS alisphenoid, or greater wing of the sphenoid; Fr frontal; Pa parietal; SO supraoccipital; M mastoid portion of the periotic; Sq squamosal; Per petrous portion of the periotic; the large foramen below the end of the line is the internal auditory meatus, the small depression above it is the nearlyobliterated floccular fossa. ExO exoccipital, the line points to the condylar foramen; fm foramen magnum; BO basioccipital; BS basisphenoid; st sella turcica; PS presphenoid, ankylosed with the basisphenoid, forming the 'body of the sphenoid;" Pt pterygoid; Pl palatine; Vo vomer; Mx maxilla; s symphysis of mandible; cp coronoid process; cd articular condyle; a angle; sh stylohyal, or "styloid process of temporal;" ch ceratohyal, or lesser cornu of hyoid; bk basihyal, or body of hyoid; th thyrohyal, or greater cornu of hyoid.

and in this young skull still separated from the basisphenoid in front by a vertical fissure. The basisphenoid (BS) is short and deep, and has a strongly marked pituitary fossa or "sella turcica" (st) above. It has completely united with the presphenoid (PS), though at birth the line of separation (below the spot called the olivary process or tuberculum sella) is still visible. In adult age large air-cells fill the interior of this conjoined bone, which is the "body" of the so-called "sphenoid" of human anatomy. Anteriorly the presphenoid narrows to a sharp vertical edge, which is in contact with the mesethmoid (ME) above and the vomer (Vo) below. The whole of the upper part of the mesethmoid is ossified in the specimen described, constituting the "lamina perpendicularis," but the anterior and lower part forms the septal cartilage of the nose. Its upper border forms a strong compressed triangular projection into the cranial cavity, called the "crista galli" (cg).

The posterior segment of the brain-case is completed, as in the Dog, by the pair of exoccipitals (ExO), and a large supraoccipital (SO).1 The triangular upper part of the latter may be considered to represent the interparietal, though it very soon becomes incorporated with the rest of the supraoccipital. The middle segment is completed by large alisphenoids (AS), the "greater wings of the sphenoid bone," and enormously extended, somewhat square shaped parietals (Pa); the frontal segment by narrow triangular orbitosphenoids (OS), the "lesser wings of the sphenoid bone," 2 and by large arched frontals (Fr).

1 The "occipital bone" of human anatomy is formed by the coalescence of the basioccipital, exoccipitals, and supraoccipital.

2 The "sphenoid bone” of human anatomy is formed by the union of the basisphenoid, presphenoid, alisphenoids, orbitosphenoids, and the pterygoids. The basal portion ultimately ankyloses with the occipital.