Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/872

Rh ANATOMY [SKELETON- The Human Spine is more uniform in length in persons of the same race than might be supposed from the indi vidual differences in stature, the variation in the height of the body in adults being due chiefly to differences in the length of the lower limbs. The average length of the spine is 28 inches ; its widest part is at the base of the sacrum, from which it tapers down to the tip of the coccyx. It diminishes also in breadth from the base of the sacrum upwards to the region of the neck. Owing to the pro jection of the spines behind and the transverse processes on each side, it presents an irregular outline on those aspects; but in front it is more uniformly rounded, owing to the convex form of the antero-latcral surfaces of the bodies of its respective vertebra. In its general contour two series of curves may be seen, an antero-posterior and a lateral. The antero-posterior is the more important. In the infant at the time of birth the sacro-coccygeal part of the spine is concave forwards, but the rest of the spine, except a slight forward concavity in the series of dorsal vertebrae is almost straight. When the infant begins to sit up in the arms of its nurse, a convexity for wards in the region of the neck appears, and subsequently, as the child learns to walk, a convexity forwards in the region of the loins. Hence in the adult spine a series of convexo-concave curves are found, which are alternate and mutually dependent, and are associaied with the erect attitude of man. In the human spine alone are the lumbar vertebrae convex forward. A lateral curve, convex to the right, opposite the third, fourth, and fifth dorsal vertebrae, with compensatory curve convex to the left immediately above and below, is due apparently to the much greater use of the muscles of the right arm over those of the left, drawing the spine in that region some what to the right. In disease of the spine its natural curvatures are much increased, and the deformity known as humpback is produced. As the spine forms the central part of the axial skeleton, it acts as a column to support not only the weight of the body, but of all that can be carried on the head, back, and in the upper limbs : by its transverse and spinous processes it serves also to give attachment to numerous muscles, and the transverse pro cesses of its dorsal vertebra? are also for articulation with the ribs. The THORAX, PECXUS, or CHEST is a cavity or enclosure the walls of which are in part formed of bone and cartilage. Its skeleton consists of the sternum in front, the twelve dorsal vertebrae behind, and the twelve ribs, with their corresponding cartilages, on each side. The Sternum or Breast Bone is an elongated bone which inclines downwards and forwards in the front wall of the chest. It consists of three parts an upper, called manubrium or pra&amp;gt;sternum ; a middle, the body or meso- sternum ; and a lower, the ensif orm process or xiphi- sternum. Its anterior and posterior surfaces are marked by transverse lines, which indicate not only the subdivision of the entire bone into three parts, but that of the meso- sternum into four originally distinct segments. Each lateral border of the bone is marked by seven depressed surfaces for articulation with the seven upper ribs : at each side of the upper border of the prse-sternum is a sinuous depression, where the clavicle, a bone of the upper limb, articulates with this bone of the axial skeleton. The xiphi-sternum remains cartilaginous up to a late period of life, and from its pointed form has been named the ensi- form cartilage. The Ribs or Costoe, twenty-four in number, twelve on each side of the thorax, consist not only of the bony ribs, but of a bar of cartilage continuous with the anterior end of each, bone, called a costal cartilage, so that they furnish examples of a cartilaginous skeleton in the adult human body; in aged persons these cartilarros usually become converted into bone. The upper seven ribs are connected by their costal cartilages to the side of the sternum, and are called sternal or true ribs ; the lower five do not reach the sternum, and are named a-sternal or false, and of these the two lowest, from being comparatively unattached in front, are called free or floating. All the ribs are articu lated behind to the dorsal vertebra?, and as they are sym metrical on the two sides of the body, the ribs in any given animal are always twice as numerous as the dorsal vertebrae in that animal. They form a series of osseo- cartilaginous arches, which extend more or less perfectly around the sides of the chest. A rib is an elongated bone, and as a rule possesses a head, a neck, a tubercle, and a shaft. The head usually possesses two articular surfaces, and is connected to the side of the body of two adjacent dorsal vertebras ; the neck is a constricted part of the bone, uniting the head to the shaft ; the tubercle, close to the junction of the shaft and neck, is the part which articu lates with the transverse process of the vertebra. The shaft is compressed, possesses an inner and outer surface, and an upper and lower border, but from the shaft being somewhat twisted on itself, the direction of the surfaces and borders is not uniform throughout the length of the bone. The ribs slope from their attachments to the spine, at first outwards, downwards, and backwards, then down wards and forwards, and where the curve changes from the backward to the forward direction an angle is formed on the rib. The first, tenth, eleventh, and twelfth ribs articulate each with only a single vertebra, so that only a single surface exists on the head : the surfaces of the shaft of the first rib are almost horizontal ; those of the second very oblique ; the eleventh and twelfth ribs are rudi mentary, have neither neck nor tubercle, and are pointed anteriorly. The ribs are by no means uniform in length : they increase from the first to the seventh or eighth, and then diminish to the twelfth ; the first and twelfth are therefore the shortest ribs. The first and second cottal cartilages are almost horizontal, but the others are directed upwards and inwards. In its general form the chest may be likened to a trun cated cone. It is rounded at the sides and flattened in front and behind, so that a man can lie either on his back or his belly. Its truncated apex slopes downwards and forwards, is small in size, and allows of the passage of tho windpipe, gullet, large veins, and nerves into the chest, and of several large arteries out of the chest into the neck. The base or lower boundary of the cavity is much larger than the apex, slopes downwards and backwards, and is occupied by the diaphragm, a muscle which separates the chest from the cavity of the abdomen. The transverse diameter is greater than the antero-posterior, and the antero- posterior is greater laterally, where the lungs are lodged, than in the mesial plane, which is occupied by the heart. The HEAD forms the summit of the axial part of the body. It consists of two portions the Cranium and the Face. The SKULL, or skeleton of the head, is composed of 22 Skull. bones, 8 of which form the skeleton of the cranium, 14 that of the face. Except the lower jaw, which is move- able, the bones are all firmly united by immovable joints. The 8 bones of the cranium arc so united together by their edges as to form the walls of a box or cavity, the cranial cavity, in which the brain is lodged. The box of the cranium possesses a base or floor, a vault or roof, an anterior, a posterior, and two lateral walls. The posterior wall is formed by the occipital bone, which also extends for some distance forwards along the middle of the base; in front of the basal part of the occipital is the sphenoid, which also sends a process upwards on each side of tho