Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/444

 398

ANATOMY

throughout the whole column consist of comparable parts modified in each region to suit the mechanical condition of the region. The most variable portion of each vertebra is the transverse process, an outgrowth from the base of the neural arch, which is usually multiple. It is not so much a morphological unit as a variably-arranged area for the attachment of oblique intervertebral muscles. In the cervical region it is tripartite—(1) its dorsal portion, existing as a mere roughness on the dorsal side of the articular mass for muscular attachment; (2) its ventral portion is fused with the rudimentai costal process ; and (3) its intermediate part, which projects into the lateral septum, is joined at its lateral end to the ventral portion, bounding with it a hole for the ascent of the vertebral artery. In the thoracic region the parts corresponding to the 1st and 3rd of these elements unite into a single transverse process, while the ventral part is flattened into a costal articular facet. The ribs, which are detached extensions of these costal processes, do not really articulate with the bodies of the vertebra, but in foetal vertebrae it can be seen that the entire rib articulation is on the neural side of the suture between the neural arch and the centrum or body. The three elements are distinct in the 12th thoracic vertebrae. In the lumbar region the ventral part is, as in the cervical, inseparable from the costal element, making with it the process that is usually erroneously described as the transverse process. The two others exist as rough spurs, called respectively the mammillary and accessory processes. The human vertebral column in its earliest stage consists of thirtyeight vertebrae. Of these the two hindmost vanish before chondrification takes place, and the four which precede these unite into an irregular mass, the last of the four pieces of the coccyx. The appendicular skeleton is described in vol. i. p. 838. The morphological nature of the vertebrate limb is still a disputed . point; but the balance of evidence is in favour of limbs6 ° anterior Balfour’sortheory, thatofeach limb ridge, is the which surviving part, posterior, a lateral originally extended along the side of the vertebrate body, and each consists of material derived from at least seven successive segments. Gegenbaur, however, has made out a plausible case for the hypothesis that the limbs are modifications of the processes of a gill arch, and Graham Kerr has proposed to regard the limbs as derived from external gills (Proc. Camb. Philosoph. Soc. 1889). The parts of the skeleton of the human forelimb present certain characters which distinguish them from those of other mammals. The scapula is longitudinally elongated and transversely short, so that its maximum breadth only equals about 0 "70 of the length. This “scapular index” is usually greater in the black races, and is higher in the foetus than in the adult. The humerus in man exceeds the radius in length as 1 :0-7, the numerical relation being known as the humero-radial index. This also is larger in the lower races. The human thumb is perfectly opposable to the other digits as is that of no other vertebrate. There are normally eight bones in the human carpus, but embryonically a ninth or os centrale is present in vestige, fusing at an early period with the scaphoid or semilunar, and only remaining distinct as a rare anomaly, although its rudiment is by no means uncommon. The pisiform bone of the human carpus is in all probability the reduced vestige of an obsolete sixth digit, and is not, as has been supposed, a sesamoid bone. The hip-bone presents markedly different characters in the two sexes, which can be recognized as early as the fifth month of fcetal life. On account of these differences the shape of the cavity of the pelvis bounded by these bones is also distinctive, even in the foetus. In the males of the white races of mankind the sagittal (antero-posterior) length of the pelvis is, in average cases, to the coronal (transverse) length as 80 :100 ; but in those of the black races it is related as 95 :100. A slight difference has been observed in the level of the articulation of the hip-bone with the sacral vertebrae at different periods of life, the tendency being for the former to rise to a higher level as development advances. In the earliest period oi foetal life at which these elements can be distinguished the first sacral vertebra is not at all involved in the articulation ; but as age advances it gradually becomes included in the joint, and occasionally the twenty-fourth vertebra becomes also on one or both sides directly articulated to the hip-bone, and correspondin°'ly incorporated in the sacrum. The femur constitutes usually about 0'275 of the individual stature but this proportion is not constant, as this bone forms a larger element in the stature of a tall than of a short man. The human femur presents also a concave popliteal surface, thus differing from that of Pithecanthropus, whose popliteal surface is convex. In the bones of some races the dorsal ridge of the thighbone (linea aspera) projects as a prominent crest causing the bone to appear “ pilastered, ” a condition the amount of which is indicated by the increased relative length of the sagittal to the coronal diameter of the bone. Pilastermg, though characteristic of lower and primitive races of man, is never found in the anthropoids. The upper third of the femur in some races is sagittally

flattened, a condition which is called platymeria. Its degree is indicated by the excess of the coronal over the sagittal diameter in this region. The tibia in most civilized races is triangular in the section of its shaft, but in many savage and prehistoric races it is twoedged. The condition is named platycnemia, and is indicated by the proportional excess of the sagittal over the coronal diameter. The fcetal tibia has its head slightly bent backwards with regard to the shaft, a condition which usually disappears in the adult, but which is shown by the prehistoric tibiae found in the cave of Spy. In races that squat on their heels the front margin of the lower end of the tibia is marked by a small articular facet for the neck of the astragalus. The fibula is a variable bone. In prehistoric and savage races its several surfaces are often deeply channelled. In foetal life its lower ankle-swelling is smaller than the inner ankle of the tibia, but this relation becomes reversed in the adult. The human foot is plantigrade, the metatarsal bone of the great toe being nearly parallel to those of the other toes, unlike those of the anthropoids. Very often in the adult the second and third phalanges of the little toe are ankylosed together. This is no new thing nor is it necessarily a product of civilization. The writer has found it thirty times in the feet of ancient Egyptians, and once in those of an aboriginal Australian. The intermembral relation of humerus and femur is important in physical anthropology, as are also the relative lengths of the femur and tibia. The varieties of cranial shape which are distinctive of the different races of mankind are due especially to the interaction of four factors—(1) brain size and shape ; (2) dental size and skull u degree of development of the muscle of mastication ; (3) ’ the sizes of the several sense organs ; and (4) the sizes and extents of certain muscular areas. The methods commonly in use for estimating these varieties are numerical indexes, which are the centesimal relations of certain measurements. These are convenient, although often fallacious. Thus, for example, brachycephaly or broad-headedness, indicated by the breadth being more than '80 of the length, may be due to the survival of an infantile character, as in Negritos, or to a secondary increase in the width of the hinder part of the frontal cerebral lobes, as in the Teutonic Europeans. Similarly dolichocephaly (with the breadth '75, or under, of the length) may be due to increased length of frontal and parietal areas of the brain (indicated by increased preauricular length), or to diminution of the parietal width and increased occipital length. Most other composite cranial measurements hitherto made are equally faulty. The human skull is frequently the subject of deformation, due to premature union of some of its sutures constraining its growth. Thus, if the two parietals unite in foetal life or infancy, the skull becomes scaphocephalic, as represented in vol. i. p. 832, Fig. 14. When the union is of the coronal suture, the skull becomes short and high, or thyrsocephalic; if the halves of the frontal unite before birth, it is wedge-shaped or trigonoccphalic. The vertebral theory of the skull has been discussed (vol. i. p. 831), and is now definitely abandoned by all morphologists. On the other hand the original segmentation of the cephalic region has been abundantly demonstrated; the only difference of opinion among observers has been in the estimation of the number of segments in the cephalic region of the primitive vertebrate. These segments, however, had lost their distinctness even before the cartilaginous, cranium had become developed, so that there is no real segmental value in the elements of this, still less in those of the bony skull. The only place in which segmental elements can be distinguished is in the occipital region, which is in structure transitional between the head and vertebral column. (For an excellent account of the human skull, in which the many new morphological details due to recent investigation are set forth, see Graf Spee in Bardelebcn's Handbuch der Anat. i. 2.) To the account of the joints and ligaments of the human body in vol. i. p. 834, it need only be added that ligaments are not all of the same morphological value. The capsules of j0iats most of the joints are derived from the connective tissue which forms the circumference of the primary articular cleft in the embryo (vol. i. p. 833, Fig. 16) ; but the accessory or lateral ligaments are often of secondary formation, derived from displaced tendinous appendages to muscles or intermuscular septa, or they may even be the connective sheaths of muscles which have lost their fleshy substance. The function of ligaments is chiefly to supplement the muscles in protecting joints. They are generally weak where the muscles are strong, and it is noteworthy that joints are more frequently dislocated at the places where the muscles are feeblest, irrespective of the strength of the ligaments. The menisci, whose formation has been referred to and illustrated in vol. i. (p. 833, Fig. 17), are not always the simple survivals of an interjected layer of the capsule ; in some cases, as in the triangular cartilage between the ulna and carpus, the cartilage may contain a vestigial skeletal element, and the same may be true of the sterno-clavicular cartilacre.