Until the age of puberty there is little difference between the skull of the female and that of the male. The skull of an adult female is as a. rule lighter and smaller, and its capacity is about 10 per cent less than that of the male.
Its walls are thinner and its muscular ridges less marked; the glabella, superciliary arches, and mastoid processes are less prominent, and the corresponding air-sinuses are small or rudimentary. The upper margin of the orbit is sharp, the forehead vertical, the frontal and parietal eminences prominent, and the vault somewhat flattened. The contour of the face is rounder, the facial bones are smoother, and the mandible and maxilla, and their contained teeth smaller. Speaking generally, more of the infantile characteristics are retained in the skull of the adult female than in that of the adult male. A well-marked male or female skull can easily be recognized as such, but in some skulls the respective characteristics are so indistinct that the determination of the sex may be difficult or impossible.
Skulls vary in size and shape, and the term craniology is applied to the study of these variations.
The capacity of the cranial cavity constitutes a good index of the size of the brain which it contained, and is most conveniently arrived at by filling the cavity with shot and measuring the contents in a graduated vessel. Skulls may be classified according to their capacities as follows:
- Microcephalic, with a capacity of less than 1350 mL – eg those of native Australians and Andaman Islanders.
- Mesocephalic, with a capacity of from 1350 mL. to 1450 mL- eg.those of African negroes and Chinese.
- Megacephalic, with a capacity of over 1450 mL – eg those of Europeans, Japanese and Eskimos.
In comparing the shape of one skull with that of another it is necessary to adopt some definite position in which the skulls should be placed during the process of examination. They should be so placed that a line carried through the lower margin of the orbit and upper margin of the opening of the external auditory meatus is in the horizontal plane. The normae of one skull can then be compared with those of another, and the differences in contour and surface-form noted. Further, it is necessary that the various linear measurements used to determine the shape of the skull should be made between definite and easily localized points on its surface. The principal points may be divided into two groups (1) those in the median plane, and (2) those on either side of it (fig. 399).
The points in the median plane are the:
- Pogonion. The most prominent point of the chin.
- Alveolar point or proahion. The central point of the anterior margin of the upper alveolar arch.
- Akanthion. The tip of the anterior nasal spine.
- Subnasal point. The middle of the lower border of the anterior nasal aperture, at the base of the anterior nasal spine.
- Rhinion. The most prominent point of the internasal suture.
- Nasion. The central point of the frontonasal suture.
- Olabella. The point in the median plane at the level of the superciliary arches.
- Ophryon. The point in the median plane of the forehead at the level where the temporal lines most nearly approach each other.
- Bregma. The meeting point of the coronal and sagittal sutures.
- Obelion. A point in the sagittal suture on a level with the parietal foramina.
- Lambda. The point of junction of the sagittal and lambdoid sutures.
- Occipital, point. The point in the median plane of the occipital bone farthest from the glabella.
- Inion. The most prominent point on the external occipital protuberance in the median plane.
- Opisthion. The mid-point of the posterior margin of the foramen magnum.
- Basion. The mid-point of the anterior margin of the foramen magnum.
The points on each side of the median plane are the:
- Gonion. The outer margin of the angle of the mandible.
- Jugal point. The angle between the temporal border of the zygomatic bone and the upper border of the zygomatic arch.
- Dacryov. The point of union of the anterosuperior angle of the lacrimal with the frontal bone and the frontal process of the maxilla.
- Pterion. This point is the centre of the circular area of the same name.
- Stephanion. The. point where the temporal line intersects the coronal suture.
- Auricular point. The centre of the orifice of the external auditory meatus.
- Supra-auricular point. A point on the posterior root of the zygomatic arch, above the middle of the orifice of the external auditor- meat-us.
- Asterion. The point of meeting of the lambdoid, masto-occipital, and mastoparietal sutures.
The horizontal circumference of the cranium is measured in a plane passing through the glabella in front, and the occipital point behind ; it averages about 50cm in the female and 52.5 cm in the male.
The length is measured from the glabella to the occipital point, while the breadth or greatest transverse diameter is usually found a little above and behind the opening of the external auditory meatus. The proportion of breadth to length; (breadth x 100 / length) is termed the cephalic index or index of breadth.
The height is measured from the basion to the bregma., and the proportion of height to length (height x 100 / length) constitutes the vertical, or height index.
In studying the face the principal points to be noticed are the proportion of its length to its breadth, the shape of the orbits and of the anterior nasal aperture, and the degree of projection of the ,jaws.
The length of the face is measured from the nasion to the lower margin of the mandible, or, if the mandible be wanting, to the alveolar point; its width is the distance between the zygomatic arches. By comparing the length with the width of the face, skulls may be divided into two groups ; dolichofacial (long-faced) and brachyfacial (short-faced).
The orbital index signifies the proportion which the orbital height bears to the orbital width, thus:
(orbital height x 100 / orbital width)
The nasal index expresses the proportion which the width of the anterior nasal aperture bears to the height, of the nose, the latter being measured from the nasion to the subnasal point, thus:
(nasal width x 100 / nasal height)
The degree of projection of the jaws is determined by the gnathic or alveolar index, which represents the proportion between the basi-alveolar and basinasal lengths, thus ;
(basi-alveolar length x 100 / basinasal length)
The dental index is arrived at by comparing the dental length (i.e. the distance between the anterior surface of the first premolar and the posterior surface of the third molar tooth of the maxilla) with the basinasal length, thus
(dental length x 100 / basinasal length)
Applied Anatomy.-The chief function of the skull is to protect the brain, and therefore those portions of the skull which are most exposed to external violence are thicker than those which are shielded from injury by overlying muscles. Thus, the skull-cap is thick and dense, whereas the squamous temporal, being protected by the temporal muscles, and the inferior occipital fossa, being shielded by the muscles at the back of the neck, are thin and fragile. Fracture of the skull is further prevented by its elasticity, its -rounded shape, and its construction of a number of secondary elastic arches, each made up of a single bone. The manner in which vibrations are transmitted through the bones of the skull is also of importance as regards its protective mechanism, at all events as far as the base is concerned. In the vault, the bones being of a fairly equal thickness and density, vibrations are transmitted in a uniform manner in all directions, but in the base, owing to the varying thickness and density of the bones, this is not so ; and therefore in this situation there are special buttresses which serve to carry the vibrations in certain definite directions. At the front of the skull, on each side, is the ridge which separates the anterior from the middle fossa of the base ; and behind, the ridge or buttress which separates the middle from the posterior fossa ; and if any violence is applied to the vault, the vibrations would be carried along these buttresses to the sella turcica, where they meet. This part has been termed the ‘centre of resistance,’ and here there is a special protective mechanism to guard the brain. The subarachnoid cavity at the base of the brain is dilated, and the cerebrospinal fluid which fills it acts as a, water-cushion to shield the brain from injury. In like manner, when violence is applied to the base of the skull, as in falls upon the feet, the vibrations are carried. backwards through the occipital crest, and forwards through the basilar part of the occipital and body of the sphenoid bone to the vault of the skull.
Fractures of the skull are best considered as affecting either the result or the base Fractures of the vault generally involve the whole thickness of the bone; but, sometimes the inner table alone is fractured, and portions of it driven inwards. In fractures of the skull. and especially in punctured fractures, the inner table is more splintered and comminuted than the outer and this is due to several causes. It is thinner and more brittle ; the force of the violence as it passes inwards becomes broken up and is more diffused by the time it reaches the inner table ; the bone being in the form of an arch bends as a whole and spreads out, and thus presses the particles together on the convex surface of the arch, i.e. the, outer table, and forces them asunder on the concave surface or inner table ; and, lastly, there is nothing firm under the inner table to support it and oppose the force.
The most common place for fracture of the base to occur is through the middle fossa, and here the fissure usually takes a fairly definite course. Shirting from the point: struck, which is generally somewhere in the neighborhood of the parietal eminence, it runs downwards through the parietal and the squamous temporal and across the petrous portion, frequently traversing and implicating the, internal auditory meatus, to the foramen lacerum. From this it may pass across the body of the sphenoid, through the sella turcica, to the foramen lacerum of the other side, and may indeed travel round the whole cranium so as to separate completely the anterior from the posterior part. The course of the fracture explains the symptoms to which fracture in this region may give rise.; thus if the fissure passes across the internal auditory meatus injury to the facial and auditory nerves may result, with consequent facial paralysis and deafness ; if the fissure extends through the semicircular ducts giddiness will be complained of, especially on turning the head sideways ; or the tubular prolongation of the arachnoid around the nerves in the meatus may be torn and thus permit of the escape of the cerebrospinal fluid, should there be a communication between the internal ear and the tympanic cavity together with rupture of the tympanic membrane, as is frequently the case: again, if the fissure passes across the sella turcica, and the mucoperiosteurn covering the under surface of the body of the sphenoid bone is torn, blood will find its way into the pharynx and be swallowed, and after a time vomiting of blood will result. Fractures of the anterior fossa, involving the bones forming the roof of the orbit and nasal cavity, are generally the result of blows on the forehead; but fracture of the cribriform plate of the ethmoid may be a complication of fracture of the nasal bone. When the fracture implicates the roof of the orbit, the blood finds its way into this cavity, and traveling forwards, appears as a subconjunetival ecchymosis. If the roof of the nasal cavity be fractured, the blood escapes from the nose. In rare cases there may be also escape of cerebrospinal fluid from the nose, should the dura mater and arachnoid have beep torn.
The bones of the face are sometimes fractured as the result of direct violence. Those most commonly broken are the nasal bones and the mandible; the latter is by far the most frequently fractured of all the facial bones. Fracture of the nasal bone is for the most part transverse, and takes place about 1.25 cm. from the free margin. The broken portion may be displaced backwards or more generally to one side by the force which produced the lesion. The zygomatic bone is probably never broken alone-that is to say, without fracture of some of the other bones of the face. The zygomatic arch is occasionally fractured, and when this occurs as a result of direct violence the fragments may be displaced inwards. Fractures of the maxilla may vary much in degree, from the chipping off of a, portion of the alveolar arch to an extensive comminution of the whole bone from severe violence, as the kick of a horse. The most common situation for a. fracture of the mandible is in the neighborhood of the canine tooth, as at this spot the bone is weakened by the deep socket for the root of this tooth; it is next most frequently fractured at the angle ; then at the symphysis ; and finally the neck of the mandible or the coronoid process may be broken. Occasionally a double fracture may occur, one in either half of the bone. The fractures are usually compound, from laceration of the mucous membrane covering the gums.
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