The head. The skull, bones of the skull, sutures, arch of the skull, base of the skull.—Measurement of the skull; facial angle, angle of Daubenton; comparison of the superficies of the skull and of the face.—System of Gall.—The face, bones of the face, upper jaw, lower jaw.
The head is the most important part of the body, and to it and to the organs which it contains our attention is most particularly attracted. The heart and lungs support life by the respiration and the circulation, the digestive apparatus nourishes the body, but the head is the seat of intelligence, the centre in which all the nervous impressions meet and from which radiates the will. In the head are united the organs of sight, of hearing, of smell, and of taste; the face, almost entirely formed by the grouping of these organs, expresses by the aid of numerous muscles the impressions transmitted to the brain, the passions, calmness or agitation of mind, and, within certain limits, the phases of thought In other regions of the body life is unconscious, and the functions in their performance, whether healthy or diseased, are executed mechanically; the head alone perceives sensations and interprets their meaning, it is by it that man knows himself, by it he feels that he lives, and is able to say, "I think, therefore I am".
The head is formed of two distinct parts: first, the skull, a bony case which envelops the brain, and incloses in the thickness of one of the bones of which it is formed the organ of hearing; secondly, the face, in which are united the organs of sight, of smell, and of taste.
The skull is composed of eight bones: the frontal or coronal, which corresponds to the forehead or sinciput; the occipital in the posterior part of the skull or occiput; the two parietal bones, which form the side walls of the skull, and contribute, with the frontal and occipital, to form its arch; the two temporal bones occupy, as their name shows, the region of the temples; the ethmoid, which owes its name to the sieve-like plate of its upper surface; and the sphenoid, so called because it is wedged in between all the other bones with which it articulates, and which rest upon it as upon the keystone of an inverted arch, thus forming the base of the skull on which the brain rests. The frontal, occipital, parietal, and temporal bones are flat, formed of two plates of ivory tissue—internal and external tables—between which is a more or less thick layer of spongy tissue.
The bones of the cranium are united by means of sutures formed by the junction of the teeth of their serrated borders, almost precisely like what is termed in architecture dovetailing. At birth the bones which form the arch of the skull are united only by a membranous tissue, and their borders overlap each other even on slight pressure in such a way as to lessen the diameter of the head; but although the sutures are not yet developed, a part of the tooth-like processes already exists in a rudimentary state. The membranous intervals are greater at the point of union of the occipital and frontal with the parietal bones; these spaces are called the fontanelles. They are soon filled with bony tissue, and at four years of age not a trace of them remains. About the end of the third year the borders of the bones are cut into fine notches or teeth, which increase in number till the period of adolescence. Before this the suture which unites the two halves of the frontal bone begins to disappear; and later still, when the brain is fully developed, the other bones gradually close together.
The internal surface of the skull presents a series of depressions, portions of the arch which have been called fossa, and according to the bones which constitute them, the frontal, occipital, and temporal fossa, and which correspond to the projections which we see on the external surface. There are also a great number of projections and depressions, which to a certain extent are modelled to the surface of the brain, but which form no relief on the outside of the skull. There is no opening in the arch of the skull, but there are several in its base through which the nerves and bloodvessels pass; the most important of these is the foramen magnum, which is the communication between the cavity of the skull and the vertebral canal.
The skull is oval in form flattened at the base, with the larger end at the back. It is never perfectly symmetrical, and differs in shape and size according to age, the individual, and the race. It is larger in proportion in the infant than in the man, and in the white race than in the other races. But whatever may be the varieties which it presents, they appear exclusively in the arch.
Starting from the principle that the skull is modelled upon the brain, in measuring its dimensions we seek to ascertain that of the organ which it incloses. To attain this object, Camper drew two straight lines, the one starting from the first incisors of the upper jaw and passing over the median line of the forehead; the other starting from the auditory canal and carried horizontally till it encountered the first, formed with it an angle called the facial angle, which is from 80 to 85 degrees in the European, 75 in the Mongolian race, and 70 in the Negro. This anatomical character, considered as an expression of intelligence, did not escape the notice of the artists of antiquity. The statues which they have bequeathed to us prove this; among their gods, the facial angle of Jupiter Trophonius, for example, is 90 degrees.
Daubenton proposed to measure the occipital angle to complete the measurement of Camper, which applied only to the anterior portion of the skull; but these angular measurements would not give the extent of a solid nor of a cavity; the thickness of the bones at certain points, and the varying development of the cavities or sinuses comprised between the internal and external tables, would take from these measurements much of their signification. In order to be more exact, Cuvier, dividing the head by a section from front to back, compared the area of the skull with that of the face, leaving out the lower jaw: he found that in Europeans, the area of the skull was four times that of the face, and in the Negro the area of the face is greater by a fifth, that of the skull being proportionally less.