Zygomatic Arch

ZYGOMATIC ARCH, arcus zygomaticus (synonyms: pons zygomaticus, s. jugum), a paired flat bony bridge formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone of the skull. The suture connecting these processes is called the sutura zygomatico-temporalis. From the standpoint of phylogenesis, the zygomatic arch is a complex formation (see Skull, comparative anatomy). In reptiles, for instance in Sphenodon (Hatteria), crocodiles, and some fossil forms, there are two zygomatic arches: upper and lower. The first is formed by the squamosal and postfrontal bones, and the second by the quadratojugal (s. paraquadratum) and jugal (zygomatic). More often, one of these arches persists, with its composition including elements of both the upper and lower arches. Such a mixed composition of the zygomatic arch is also observed in mammals: posteriorly the squamosal bone, and anteriorly the quadratojugal and jugal bones. Shrews and some insectivores lack a true zygomatic arch, having only a freely terminating zygomatic process (from the squamosal bone); others, such as anteaters and sloths, have a zygomatic process connected to the jugal bone by a special ligament. Finally, the zygomatic arches undergo complete reduction, as observed in all snakes, lizards, and some turtles. In the primordial skull (see) of man, the zygomatic arch appears as a membranous formation; later it is replaced by investing bones (processes of the zygomatic bone and the squamous part of the temporal bone). Both zygomatic arches are attached to the skull like horizontal handles on a pot, hence their ancient name ansae capitis (J. Hyrtl). When looking at the human skull from below, the arches appear to lie freely; upon examination from above, they are scarcely noticeable or completely covered by the braincase. In adult anthropoid apes, due to the weak development of the cerebral part of the skull and the powerful development of the masticatory apparatus, the zygomatic arches protrude distinctly as "pot handles" (Braus). The same is true for the majority of mammals. Thus, a wide gap remains between the zygomatic arch and the braincase, communicating in many mammals with the orbital cavity—the so-called temporal fossa, the outer wall of which is formed by the zygomatic arch. The thickness, strength, and relief of the zygomatic arch are related to the degree of development of the masticatory apparatus and, in particular, the masseter muscle attached to it. The temporal fascia is attached by two laminae to the upper margin of the lateral and medial surfaces of the zygomatic arch; the parotideo-masseteric fascia is attached to the lower margin. The superficial temporal artery (accompanied by the vein of the same name) passes across the zygomatic arch near the anterior margin of the auricle, and its pulsation at this site can be felt on the living subject. Here, this artery can also be compressed against the bone and exposed for ligation. The curvature of the zygomatic arches—the degree of their protrusion—characterizes the width of the face (the facial part of the skull). The most laterally protruding point on the lateral surface of the zygomatic arch is called zygion in anthropometry; the distance between the right and left points is the zygomatic diameter. The zygomatic arches have a significant influence on facial proportions. The facial index is determined by the ratios of the zygomatic diameter to the height of the face. The influence of the zygomatic arches on facial proportions is greater during the period before the eruption of deciduous teeth and the formation of the upper jaw than at older ages. The degree of protrusion of the zygomatic arches, along with the position of the zygomatic bones (whether more frontally or sagittally oriented), is a characteristic racial feature. The zygomatic arches, as distinct bony landmarks, are used in constructing diagrams of so-called craniocerebral topography (see). V. Uskov.