Danger and death are symbolized by the skull and crossbones, which is often found on labels that warn of poisonous contents. The skull and crossbones was also adopted by pirate ships and the Hell’s Angels. Each Halloween the skull—with or without the crossbones—is still used to warn of danger and death.
(also cranium), the skeleton of the head of vertebrates, including man. A distinction is made between the axial cranium and the visceral cranium. The axial, or cerebral, cranium is the anterior continuation of the axial skeleton of the trunk that grows around the brain, olfactory organs, and inner ear. The visceral, or facial, cranium is the skeleton of the anterior part of the intestine (pharynx), which originally consisted of branchial, or visceral, arches separating gills.
In animals. The cranial changes that evolved in organisms were caused by progressive development of the brain and sense organs, replacement of gill respiration by pulmonary respiration, and change in feeding habits resulting from the emergence of organisms onto land from water. The cerebral cranium consists of a brain case, nasal capsules (surrounding the olfactory organs), and auditory capsules (enclosing the inner ear). The brain case is divided into anterior (prechordal) and posterior (chordal) portions. The former embraces the orbital and nasal regions of the skull, and the latter embraces the occipital and aural portions. The anterior portion develops from trabecular cartilages and orbital cartilages situated above them. In embryos the posterior portion develops around the fore end of the chord from the parachordal cartilages and auditory capsules. The parachordal cartilages correspond to the fused neural arches of the most anterior vertebrae, hence the “vertebral” origin of the posterior portion. Both portions remain independent in crossopterygians but grow together in the embryos of other vertebrates. The anterior portion usually contains only the forebrain (the cerebral hemispheres), with the posterior portion containing the greater part of the brain. In the adult cranium the boundary between the portions passes through the hypophysis and the opening by which the trigeminal nerve emerges from the cranium. The nasal capsules fuse with the anterior portion. Crossopterygians and terrestrial vertebrates have, in addition to nostrils, internal nares (choanae) that open into the anterior part of the oral cavity.
Platybasic and tropibasic types of brain case structure are distinguished according to the structure of the orbital portion. In a platybasic skull the prechordal portion has a wide base, and the brain extends into the orbital region. This type of cranium characterizes cyclostomes, sharks, lungfishes, recent amphibians, and mammals. All other vertebrates have a tropibasic skull, whose anterior portion has a narrow base and whose cerebral hemispheres are situated behind the orbital region.
The visceral cranium serves as a support for the anterior part of the digestive tract. It consists of metamerically arranged cartilaginous arches that encircle the initial portion of the digestive tract like hoops. The anterior visceral arches are transformed into jaws in all vertebrates except the most primitive organisms—the Agnatha. The cartilaginous base of the upper jaw, the palatoquadrate cartilage, was formed by the fusion of the upper portions of the two anterior arches, the premaxillary and the maxillary proper. The cartilaginous base of the lower jaw—Meckel’s cartilage— corresponds to the lower portion of the second visceral, or maxillary, arch. The upper portion of the third visceral arch, or hyoid arch, was transformed in fish into an appendage that usually attaches the jaws to the axial cranium. In most fish the maxillary arch is joined to the axial cranium only by an appendage (hyostylic). The palatoquadrate cartilage in chimeras, lungfishes, and terrestrial vertebrates is directly joined to the axial cranium without an appendage (autostylic). In extinct shelled and primitive bony fishes and some sharks the maxillary arch is joined to the axial cranium both directly and by means of an appendage (amphistylic).
In terrestrial vertebrates, the appendage is transformed into an auditory ossicle that conducts sounds from the tympanic membrane to the inner ear; the posterior visceral arches (gill arches proper) are transformed into the hyoid bone and laryngeal cartilages. Mammals have additional auditory ossicles, the incus and the malleus, which arose from detached posterior elements of the palatoquadrate and Meckel’s cartilages—the quadrate and articular bones.
In higher fishes and terrestrial vertebrates, the embryonic cartilaginous cranium more or less ossifies and becomes invested with dermal bones formed in the deep layers of the skin. On the outside the dermal bones form the calvaría, which is made up of nasal, frontal, parietal, squamous, and other bones. Originally the calvaría had openings only for the eyes and nostrils (anapsid skull), but most terrestrial vertebrates now have openings behind the orbits—temporal windows separated by the temporal, or zygomatic, arches. The function of the upper jaws was transferred to the premaxillary and maxillary dermal bones. The palatal and pterygoid dermal bones of the palatoquadrate cartilage form the palatal surface of the bony cranium. In some reptiles and mammals these bones form a secondary palate, which separates the choanal region from the main part of the oral cavity.
L. P. TATARINOV
In man. The human cerebral cranium is made up of six flat curved bones: the unpaired frontal and temporal bones, the paired parietal and temporal bones, and the sphenoid and ethmoid bones (situated at the base of the skull and possessing air spaces). The bones are connected by sutures. The upper part of the skull is called the skullcap, and the lower part is the base. The latter is divided from within into three fossae: anterior, middle, and posterior. The posterior cranial fossa contains the cerebellum. Nerves and blood vessels pass through the base of the skull by way of numerous canals and openings. The ear is housed in the temporal bone.
The shape and size of the skull vary: the circumference ranges from 52 to 64 cm, the length from 15 to 18 cm, and the width from 12 to 15 cm. Three basic shapes have been determined according to the cranial index. The shape of the skull changes with growth. An infant is born with incompletely developed bones, between which are remnants of the membranous cranium in the form of fontanelles and wide sutures.
The facial cranium consists of 14 comparatively small bones and the hyoid bone. The largest of these bones are the upper and lower jaws, which bound part of the oral cavity. The bones of the facial cranium as a whole determine the shape of the face. The lacrimal, zygomatic, and maxillary bones contribute to the formation of the orbit. The nasal, palatine, maxillary, vomer, and inferior turbinate bones form the nasal cavity, which communicates with the frontal, maxillary, and sphenoidal air sinuses. Other important topographical formations of the cranium include the infratemporal and pterygoid fossae, through which blood vessels and nerves pass.
V. V. KUPRIANOV
A. K. VLADIMIROVA