842 ANATOMY [ANATOMY OF limbs. Suppose a person to be standing erect, with one leg a little in advance of the other ; the body, being- inclined slightly forwards, is pushed in advance by the extension of the hindmost limb, so that the weight falls more and more upon the advanced leg, which at the same tune is shortened by bending the knee and ankle. The heel of the hindmost limb being then raised by the action of the muscles of the calf, the toes press against the ground so as to push the trunk so far in front of the advanced limb as to be no longer safely supported by it ; the hind most limb is then raised from the ground by muscular action, and allowed to swing forward by its own weight, but guided by the muscles, until the toes touch the ground in front of the opposite limb. A step has now been made, and the limbs are in a corresponding but opposite position from that in which they were when the step commenced : a repetition of the act constitutes another step, and so. the alternate action continues. At one moment in each step both feet touch the ground at the same time, i.e., when the hind foot presses against the earth. The act of running consists in a repetition of the movements of walking per formed with so much greater rapidity that the feet never touch the ground at the same moment ; the heels also are never brought to the ground. The propulsive action is also greatly increased by the extension of the hip and knee joints, so that a succession of small leaps on to alternate feet takes place. In leaping from the standing position the j oints of both lower limbs, previously flexed, are suddenly and simultaneously extended, and the body is projected forwards with a rapid impulse. Development and Homologies of the Voluntary Muscular System. The voluntary muscles, like the bones and joints with which they are so intimately associated, arc developed out of the middle of the three layers the mcso-blast into which the germinal area or blaslodcrm of the young embryo is divided. The muscles of the axial skeleton are capable of subdivision into a group situated outside the endo-skeleton, i.e., between it and the integument which muscles have recently been called epi-sJccletal and a group lying on the ventral surface of the vertebral bodies and within the rib arches, which have been termed the hcemal or hypo-skeletal muscles. The epi-skeletal muscles, like the vertebrae themselves, are developed within the jwoto-vcrtcbrcc, but it is not known if the hypo-skeletal group have the same origin. In fishes the epi- skeletal muscles preserve their fundamental arrangement with but little modification. They are disposed in transverse segments or myotomcs, which equal in number the vertebrae. These myotomes are separated from each other by bands of fibrous tissue, the inter- muscular septa. In man and the higher vertebrates the simple transversely segmented arrangement is to a large extent lost. Traces are preserved, however, in the iuterspinale.-s and iutertransversales muscles, situated in the intervals between the spines and transverse processes of some of the vertebral segments ; in the external inter- costals and in the recti abdominis muscles, in the last-named of which tendinous bands subdivide the muscle into several transverse segments. More usually, the intermuscular septa either are not formed or disappear, and adjacent myotomcs become blended into a continuous mass of muscle. In some instances the fibres of this muscle run longitudinally, and the entire mass subdivides longi tudinally into separate and distinct parallel muscles, as is seen in the subdivision of the great erector spinaj into the sacro-lumbalis, musculus accessorius, cervicalis ascendens, longissimus dorsi, trans- versalis cervicis, trachclo-mastoid, and spinalis dorsi muscles. In other instances the muscles run obliquely ; some on the back of the body pass obliquely from below upwards and outwards, as the splenius and obliquus inferior ; others obliquely from below, up wards and inwards, as the complexus, obliquus superior, semi- spinalis, multifidus and rotatores spime ; others again, as the external and internal oblique muscles of the abdomen, extend obliquely from behind forwards to the ventral mesial line. Of the hypo-skeletal group of muscles, the internal intercostals dis play the transverse segmentation. As a rule, however, the muscles of this group extend longitudinally, and form the prse-vertebrai group, named anterior recti, longi colli, and psose ; though the diaphragm, triangulares sterni, transversi abdomiuis, and levatores ani, which lie in relation to the inner surfaces of the ribs and visceral cavities, are not longitudinal, but are specially modified in arrangement for functional reasons. The plane of demarcation between the hypo- and epi-skebtal groups of muscle"; where they form together the walls of the great visceral cnambers, the thorax and abdomen,, is marked off by the position and course of the intercostal series of spinal nerves. The muscles of the appendicular skeleton are either limited to the limbs (purely appendicular, therefore), or pass from the axial part of the body to the limb (axi-appendicular). The axi-appen- dicular group are undoubtedly prolongations of the axial system of muscles. They are in the upper limb derived from the epi-skeletal subdivision, and form the traptv.ius, rhomboid, levator anguli scapulae, latissimus dorsi, serratus magnus, greater and smaller pectorals, and subclavius muscles of each superior extremity. In the lower limb they are in, part derived from the hypo-skeletal subdivision, and form the psoas and pyriformis ; and in part, as the glutaeus inaximus, from the epi-skeletal subdivision. It is not improbable that the purely appendicular muscles are also prolon gations of the axial system, and that as the limbs, in their develop ment from their fundamental bud-like lappets, undergo both a transverse and a longitudinal segmentation, so the muscular mass, prolonged into them, differentiates both transversely and longi tudinally into a motor apparatus, fitted for the performance of the special functions of each extremity. ANATOMY OP THE TEXTURES on TISSUES. Introductory. Before proceeding to the description of the other organic systems of which the human body is built up, it may be well to enter into the consideration of the minute or microscopic structure of its constituent parts. These parts may primarily be divided into fluids and solids. The fluids are the blood, the lymph, the chyle, the secretions of the various glands, and of the serous and synovia! membranes. The solids form ths framework of the several organic systems, and assume different appearances in different localities. Sometimes they are arranged in compact solid masses, as in cartilage ; at others they are elongated into fine threads or fibres, as in muscle, tendon, nerve ; at others they are expanded into thin membranes, as in the fascise or aponeuroses, the serous, synovia], and mucous membranes ; at others they are hollowed out into distinct tubes for the conveyance of fluids, as in the blood-vessels, the lymph and chyle vessels, and the ducts of glands. To the solids of the body, whatever their form may be, the general name of Tissues or Textures is applied. Each organic system may be regarded as in the main composed of a tissue or texture peculiar to and characteristic of it self. Thus, the bones are essentially composed of the osseous tissue, the muscles of the muscular tissue, the nervous system of the nervous tissue, fibrous membranes of the fibrous or connective tissue, &c. But though the essential constituent of each organic system is a tissue peculiar to that system, yet in most localities certain other tissues are mingled with that which is to be regarded as the characteristic texture of the part. In a muscle, for example, not only is the muscular tissue present, but mingled with it are connective tissue, nerve tissue, blood vessels, and lymph-vessels. A gland also not only consists of its proper tissue, the secreting cells, but of more or less connective tissue, nerves, blood and ]ymph vessels, and gland ducts. Indeed, -there are few localities in which, along with the proper tissue of the part, blood and lymph vessels, nerves and connective tissue, are not found; and to a part built up of two or more tissues the name of an Organ is applied. Thus the muscular system consists of the series of organs which we call the muscles, the gland ular system of the several organs called glands, and so on. Each tissue and each organ, into the construction of which that tissue enters as the characteristic< texture, possesses not only distinctive structural, but also distinctive functional properties. Thus the muscular tissue is characterised by the property of contractility, and the muscles, of which it forms the essential texture, are organs of motion or
locomotion ; the osseous tissue is characterised by its
