-830 ANATOMY [SKELETON- the tibia. The sliaft is three-sided, and roughened for the origins of muscles; along the inner surface is a slender ridge for the attachment of the interosseoiis membrane. The lower end has a strong process (external malleolus) projecting downwards to form the outer prominence of the ankle, and possesses a smooth inner surface for articulation with the astragalus, above which is a rough surface for the attachment of ligaments which bind together the tibia and fibula. oot. The Foot consists of the Tarsus, the Metatarsus, and the five free Digits or Toes, which is the maximum number found in mammals. The human foot is placed in the prone position, with the sole or plantar surface in relation to the ground ; the dorsum or back of the foot directed upwards ; the axis of the foot at about a right angle to the axis of the leg ; and the great toe or hallux, which is the corresponding digit to the thumb, at the inner border of the foot. The human foot, therefore, is a pentadactylous, plantigrade foot. arauy. The bones of the Tarsus, or Ankle (Fig. 1 1, Tr), are seven in number, and are arranged in two transverse rows, a proximal, next the bones of the leg, consisting of the astragalus, os calcis, and scaphoid ; a distal, next the metatarsus, consisting of the cuboid, ecto-, meso-, and ento-cuneiform. If the tarsal bones be looked at along with those of the metatarsus and toes, the bones of the foot may be arranged in two longitudinal columns, an outer, consisting of the os calcis, cuboid, and the metatarsal bones and phalanges of the fourth and fifth toes ; an inner column consisting of the astragalus, scaphoid, three cunei form, and the metatarsal bones and phalanges of the first, second, and third toes. The tarsal, like the carpal bones, are short and irregularly cuboidal ; the dorsal and plantar surfaces are as a rule rough for ligaments, but as the astragalus is locked in between the bones of the leg and the os calcis, its dorsal and plantar surfaces, as well as the dorsum of the os calcis, are smooth for articu lation; similarly, its lateral surfaces are smooth for articulation with the two malleoli. The posterior surface of the os calcis projects backwards to form the prominence of the heel. With this exception, the bones have their anterior and posterior surfaces smooth for articulation. Their lateral surfaces are also articular, except the outer surface of the os calcis and cuboid, which form the outer border ; and the inner surface of the os calcis, scaphoid, and ento-cuneiform, which form the inner border of the tarsus. A supernumerary bone is sometimes found in the human tarsus, from a subdivision of either the ento-cunei form, astragalus, os calcis, or cuboid into two parts. In some rodents and other mammals eight is the normal number of bones in the tarsus. oee. The Metatarsal bones and the Phalanges of the toes agree in number and general form with the metacarpal bones and the phalanges in the hand. The bones of the great toe or hallux are more massive than those of the other digits, and this digit, unlike the thumb or pollex, does not diverge from the other digits, but lies almost parallel to them. Development and Ilomoloyies of the Skeleton. It will now be advisable to consider briefly the mode of develop ment of the skeleton, and along with the study of its genesis to compare its several parts with each other, in order to ascertain if correspondences in their arrangement and mode of origin exist, even if they differ in the function or office which, they perform. When two or more_parts or organs correspond with each other in structure, relative position, and mode of origin, we say they are homologous parts, or homologucs; whilst parts which have the same function, but do not correspond in structure, relative position, and mode of origin, are analogous parts, or analogues. Homologous parts have therefore a morphological identity with each other, whilst analogous parts have a physiological agreement. The same parts may be Doth homologous and analogous, as the fore-limbs of a bat and a bird, both of which, with the same fundamental type of structure, are subservient to flight. In other cases analogous parts are not homologues, as is illustrated by the wing of the insect, which, though subservient to flight, is fundamentally different in structure from the wing of the bat or bird. In the germinal area of the fertilised vertebrate ovum a longitudi nal groove appears which marks the beginning of the cranial cavity and spinal canal of the young embryo. At the bottom of this cranio- sjrinal groove a slender rod is formed, called chorda dorsalis or noio- chord. Each side of the groove then becomes elevated as a thin mem brane, to meet behind to enclose a cr.nal in which the brain and spinal marrow are developed. Small dark masses, the primordial or j)t~cto- vertebrae, next form on each side of the chorda dorsalis. In these proto-vertebrre, about the sixth or seventh week of intra-uti-rim: life of the human ovum, little masses of cartilage appear, which correspond in number and position to the future spinal vertebne. The part of the cartilage which forms the body of the future vertebra is developed around the chorda dorsalis, which it encloses in itc substance, whilst the cartilaginous neural arch forms in the mem brane which closes in the spinal canal. The formation of these cartilaginous vertebrae is completed in the human embryo about the fourth month of intra-uterine life. The bodies of the cartilaginous vertebrae are connected together, by plates or discs of intervening fibre-cartilage, which are also developed around the chorda dorsalis. After the enclosure of the rod-like chorda by the cartilaginous vertebrae and the inter-vertebral discs it disappears, no remains being found in the adult human body, or in that of the higher vertebrates, except perhaps some slight traces in the soft pulpy centres of the inter-vertebral discs ; although in the cartilaginous iish it remains as a more or less complete structure throughout life. In each of the cartilaginous vertebrae bone begins to form and to spread beyond its original point of formation, which is called a centre or nucleus of ossification ; the greater part of the body is formed from one of these centres, and each half of the neural arch from another ; whilst small ossific centres arise for the tips of the spiuous, transverse, and mammillary processes, and a special plate appears for both the upper and lower surfaces of the body ; the fusion of the various centres together to form a complete vertebra takes place between the twentieth and twenty-fifth year. The atlas has a separate centre for each lateral mass and one for the anterior boundary of the ring. The axis, in addition to the ossific centres found in the vertebra} generally, has one or two for the odontoid process. The seventh cervical vertebra has the anterior bar of its transverse process developed from a separate centre. Each coccygeal vertebra possesses only a single centre, which represents- the body of the bone. At the time when the cranio-spinal canal is being closed in by the development of its membranous walls, the germinal layers of the young embryo grow towards its anterior or ventral surface, and meet in the ventral mesial line, so as to enclose the cavities in which the thoracic and abdominal viscera are developed. In the membranous wall on each side of the thoracic cavity twelve cartilaginous rods, the future ribs, are developed ; and, connected with the anterior ends of the seven pairs of upper ribs, the cartilaginous sternum is formed. Each rib ossifies from one centre for its shaft, and one each for the head and tubercle. The sternum ossifies in transverse segments, one for the prse-sternum, one or sometimes two for each of the four subdivisions of the meso-sternum, and one for the xiphi-sternum. The complete ossification and fusion of the different parts of the sternum into a single bone does not take place until an advanced age. The axial part of the skeleton, formed by the vertebrae, ribs, and sternum, is built up of a series of thirty-three transverse segments, equal in number, therefore, to the bones of the spine ; so that each vertebra, according as it is, or is not, articulated with a pair of ribs and a segment of the sternum, constitutes a complete or incomplete transverse segment. These several segments are serially homologous with each other, but the homology is not so complete ia some of the segments as in the others. In the coccygeal, sacral, and lumbar regions of man and most vertebrates, only the verte bral portion of each skeletal segment is represented, though in the abdominal wall of the crocodiles abdominal ribs and a sternum are developed. In the thoracic region the five lowest dorsal vertebne have five pairs of ribs developed in connection with them ; whilst the seven highest vertebrae have not only their corresponding pairs of ribs, but also a sternum, which bone, however, has only six trans verse segments. In the cervical region seven vertebrae are found, but the anterior bar of the transverse process, although fused with the vertebral body, is homologous with a rib, for in man it some times develops as a distinct movable rib in connection with the seventh cervical ; and in the crocodiles small movable ribs are regularly developed in connection with the different cervical verte brae. The bodies and neural arches of the vertebra are serially homologous with each other ; as a rule this is also the case with their processes, but the articular processes of the atlas and the superior pair of the axis, although functionally analogous, are not homologous with the articular processes of the other vertebrae, but
with the articular surfaces for the ribs on the bodies of the dorsal
