Portions of the bones of the cranium; showing the corresponding inequalities in their margins: which margins, when in apposition, constitute the mode of union termed suture. 1. External surface of the bone; 2. internal surface.] [Illustration: Fig. XLI. 1 Fig. XLII. 2 1. Side view of the adult skull, showing the several bones united by suture; 2. side view of the fœtal skull, showing the bones imperfectly ossified, separated to some extent from each other, the interspace being occupied by membrane. The small size of the face compared with that of the cranium is strikingly apparent.] 63. These bones are firmly united together. The union of bones is technically called an _articulation_ or _joint_. All joints are either immoveable or moveable. The union of the bones of the cranium affords an example of an immoveable articulation. Prominences and indentations, like the teeth of a saw, are formed in the margins of the contiguous bones (figs. XXXVIII. and XL.). At these inequalities of surface, which are exactly adapted to each other (figs. XXXVIII. and XL.), the two bones are in immediate apposition in such a manner as to preclude the possibility of motion, and even to render the separation extremely difficult. This mode of articulation is termed a _suture_. There are certain advantages in constructing the cranium of several distinct bones, and in uniting them in this peculiar mode. 1. The walls of the vault are stronger than they could have been had they been formed of a single piece. 2. In the fœtus, the bones are at some distance from each other (fig. XLII.); at birth, they yield and overlap one another; and in this manner they conduce to the security and ease of that event. 3. Minute vessels pass abundantly and securely through the interstices of the sutures to and from the interior of the cranium; in this manner, a free communication is established between the vessels within and without this cavity. 4. It is probable that the shock produced by external violence is diminished in consequence of the interruption of the vibration occasioned by the suture; it is certain that fracture is prevented by it from extending as far as it would do in one continued bony substance. [Illustration: Fig. XLIII. Section of the skull. 1. Cavity of the cranium occupied by the brain; 2. cut edge of the bones of the cranium, showing the two tables of compact bone and the intervening spongy texture called diploë.]