HEAD, the anterior or front part of the body of an animal when it is marked off by a difference in size, or by a constriction. The presence or absence of a head was formerly much used as a character in classification. But this line of classification is artificial. The mouth and principal nervous organs are the guides to the anterior end of the body, where the head, when recognizable, is situated. In the protozoa, infusoria and coelenterates, such as the hydra and corals, there is no nervous ganglion, and the mouth is not surrounded by special structures. In the inferior worms the front end becomes marked by the presence of ganglia. The so-called head of parasitic animals, such as the tapeworms, is only the end of attachment, but neither mouth nor ganglia exist in it. In the polyzoa, lampshells, ascidians, and lamellibranch mollusks mouth and ganglia exist, but they are not surrounded by special structures. But in the worms proper, the articulated animals, the land and fresh-water gasteropods and the cuttlefishes a. head proper their character; as the brain becomes specialized, so does the brain-case or skull. In man the brain attains its highest development and the head its greatest complexity, the difference between skull and face being now most pro nounced. The vertebrate theory of the skull, first propounded by Goethe, is now accepted to this extent, that the skull or cranium con sists of three vertebra, which are recognizable in 'the fish, and that the facial bones are not vertebra, but developed from cartilage which did not form an original part of the vertebral column. A vertebra consists of a body or centre, from which two processes arch upward and close in the spinal canal with its contents, the spinal cord. The posterior cranial vertebra is the occipital, consisting of a centre, two lat eral pieces, and a superior, the next is the parietal, of which the basisphenoid is the centre, and the great wings of the sphenoid and the parietals the lateral arches; the most anterior is the frontal, with its centre, the presphenoid, and its arch, formed by the orbital plates of the sphenoid and the frontals. The centres of the spinal vertebra are ossifications around a fibro cartilaginous rod, the chorda dorsalis, which ends in the basisphenoid. So far spinal column and skull have a common base; but the spinal vertebra were preceded by and are in fact modifications of primitive vertebra, and no rep resentatives of these appear in the development of the skull. It is therefore open to question whether the three divisions just mentioned are really vertebra, or should not rather be called cranial segments. There is the more reason is found. That is, the mouth and the anterior nervous ganglia are placed in a segment of the body which, by structure, is different from the rest. Thus in the worms and articulated animals some of the rings or articles of which the body is made up are fused together, the appendages being not walking limbs, but modified into jaws or jawlike organs. Thus the common shore worms possess a structural head, though it is not apparent. The head is first best defined in the insects. The snail's head has its cavity shut off by a diaphragm from the rest of the body cavity. The cuttlefishes have, in addition, a re
markable cartilaginous box, which like a skull, protects the ganglia and gives support to the tanscles. The head of the vertebrated animals presents a regular series of increasing com plexity from the amphioxus upward. In that fish the most anterior part of the nervous cord is lodged in a canal scarcely distinct from that which contains the rest of it. Ascending in the series, it becomes evident that as the anterior nervous mass enlarges, and its ganglia increase in complexity, the anterior vertebrae change for this that in fishes the basisphenoid and pre sphenoid are represented by a single bone, the parasphenoid, which underlies the skull, but disappears in the higher vertebrates, and that the presphenoid is not properly connected with the chorda dorsalis, but rather belongs to the series of facial bones. The pituitary body which projects from the lower surface of the brain lies in front of the end of the chorda dorsalis: from the latter rod and its surroundings a plate of cartilage passes forward on either side of the pituitary body, and these (the trabeculce), meet ing in front of that body, form the cartilaginous axis around which the vomer, ethmoid, and other facial bones are developed, while the presphenoid is an ossification in this axis just where the two portions meet in front of the pituitary. The sense organs, the ear and the eye, are, 'so to speak, lodged in capsules of bone which are inserted, the ear between the occipital and parietal, the eye between the pari etal and frontal segments. They are accidental, not essential parts of the cranium. The hyoid apparatus and the lower and upper jaws are developed from the cartilaginous walls of the embryonic skull, and the jaws come in a sec ondary manner to take part in the composition of the face. (See RESPIRATORY ORGANS). The increasingly globular form of skull in the verte brates is due to the greater increase of the cerebral hemispheres relatively to that of the base of the brain and axis of the skull; hence the brain comes in man to overhang the face. Of course it is to be remembered that while in the vertebrated animals the head is divided by its axis (commencing at the middle line of the upper jaw and passing backward through the basisphenoid to the vertebral centres) into an upper chamber, lodging the brain, and a lower, lodging the first part of the alimentary canal, in the lower animals the cavity is a single one, the oesophagus piercing the nervous system so as to reach the surface of the body, and thus corning to be surrounded by a pair of ganglia above and a pair below, with the fila ments connecting these ganglia. In the verte brate the head is curved downward, the bag sphenoid being the pivot point, so that the mouth is pushed to the lower surface; in the lower animals the under surface of the body curves upward, so as to carry a part of the nervous system past the mouth toward the upper surface. The eyes and feelers of a crab are in fact modified limbs which are thus car ried upward; the and sense organs of a vertebrate are entirely distinct from the limbs and other appendages of the trunk.
See DYAKS.