anatomy
399
II. Muscular System (see vol. i. p. 834 sq.'). The volun- passing down in the line of the middle finger, whereas in the foot the axis from and to which the muscles draw the tary muscles of the body may be grouped in three series those derived from the muscle plate of the protovertebrse, toe passes down in the line of the second toe. The muscles which are derived from the nephromeric those originating in the somatopleure ; and those originating in the nephromere or region of the segmental organs. region, sometimes called hypaxial or hyposkeletal muscles, The muscles arising in the splanchnopleure are the un- are a few, small aberrant somatopleural slips which lie on the ventral face of the vertebral bodies and pedicles, such striped muscles of the viscera. from the muscle plates arise the axial muscles of the dorsal as the longus colli and others on the same horizon. They series, including the deeper layers of the muscles of the back. are innervated by branches from the ventral divisions of The orbital muscles also originate from cephalic muscle the segmental nerves. The muscles of the human body exhibit very .many plates. In the trunk this series is trilaminar, including varieties, an exhaustive study of which is given in Le {a) the longitudinal muscles of the spinal column, (b) the Double’s Variations du systeme musculaire de Vhomme, intermediate'oblique series of semispinales and multifidi, and (c) the deep, short interspinal and rotator muscles, 1897. III. The Nervous System.—Considerable advances in which stretch from vertebra to vertebra.. All these are knowledge and some changes in interpretation have, been innervated by the dorsal branches of spinal nerves, and made since the writing of the remarks in vol. i. pp. they are limited dorsally by the strong dorsal fascia, and 858-884. The vertebrate body in its early embryonic ventrally by the lateral intermuscular septum. The somatopleural muscles originate in the body wall stages is made up of a succession of segments, in each of beyond the edge of the muscle plate. As it is from this which there is a segmental nerve distributed to the parts structure that the limbs are outgrowths, these muscles of the segment. In process of development irregularities the growth of the component elements obliterate or may be divided into two series—those of the trunk (ventro- in axial), and those of the limb (appendicular). They are obscure the definition of the primitive division; but there reason to suppose that the nerve constituents are more all innervated from the ventral branches of the spinal is constant in arrangement, and furnish more accurate indinerves and lie on the ventral side of the lateral septum. cations of the original segmentations than do the other These muscles are divided into laminae. A supeificial tissues or organs. In the head, where the traces of original external oblique layer is continued in the thoracic region segmentation are practically obliterated, owing, to the over the ribs. A second or internal oblique layer becomes rapidity with which change succeeds change in early intercostal in the thorax. A third or transverse layer lies embryogeny, almost our only guide to unravel the morphoon the visceral side of the ribs. On either side of the logical history is that furnished by the cranial nerves. medio-ventral line these three layers are represented by a continuous mass, the rectus abdominis, from which, some- The course of a typical segmental nerve is described in i. p. 867. Such a nerve traced centripetally resolves times, the pyramidalis is a superficial and partial delamma- vol. itself into two roots, a dorsal and a ventral, close to its tion. Occasionally there is a superficial lamella attached place of connexion with the spinal cord. The fibres to the skin—the panniculus, or skin muscle—which is only making up the ventral root enter the spinal cord along a delamination of the superficial layer, as shown by its its antero-lateral groove, are continued into the grey matter development and innervation. The limb muscles are continuations of the supracostal stratum, modified by their of the anterior horn, and prove to be the axons of the branching cells of that horn. These neurones form the connexions with the bony parts of the limb. They are large efferent or motor system whose terminal tufts are enclosed throughout divisible into a dorsal and a ventral series, in the fibres of the voluntary muscles and other structures each of which is innervated by branches derived respec- throughout the body. Some of the efferent axons of a tively from posterior and anterior divisions of the ventral spinal nerve end in tufts beside the dendrites of nerve-cells trunks of the spinal nerves of the segments represented m clustered in small visceral ganglia, which are arranged, in the limbs. Those who hold the Gegenbaur or Graham a chain on the ventral surfaces of the vertebral bodies. Kerr hypothesis regard the multiple innervation of the To these the name sympathetic ganglia is given. The limb as evidence of the secondary annexation of other axons of these cells are distributed to viscera and vessels. nerves, in consequence of a gradual slipping backwards of The fibres making up the dorsal root are in essence the limb. These dorsal and ventral muscles of. the fimb processes of cells which form a series of ganglia close to are grouped into those attached to the limb gir e an the postero-lateral groove of the cord. Each cell of these those which extend to the first, second, or to the terminal ganglia gives off a short process, which divides into two elements of the limb ray. There is a certain general branches that diverge from each other nearly at right resemblance between the muscles of the two limbs as they angles, like the component halves of the horizontal stroke ian arise from similar muscles-sheets; but as in ' of the letter T. One of these is the fibre we have traced animals the fore and hind limbs are of necessity different from the periphery as the sensory element of the spinal in their mechanical relations, the role of the fore and hind nerve; the other passes into the spinal cord, there to break limbs being essentially different in quadrupedal progres- up and to be connected synaptically with the neurones of sion, these sheets from the first have undergone cleavage other systems. The primordial segmentation of the veitein different ways. Therefore, it is not possible Piecis® £ brate nervous centres has become overlaid and masked by to homologate the different muscles of one limb wi the development of groups of neurones, which combine, their equivalents in the other. The innervation of these co-ordinate, and otherwise influence the relations of the sheets of muscle, however, shows that around the shoulder segmental nerve-cells. This is true of the spinal cord, or hip there are four groups, which may generally be called but much more so of the brain. abductor, adductor, flexor, and extensor; m those around As an account of the spinal cord has been given in vol. the elbow as well as in those of the hand or oo ere ar i. p. 865, it is only necessary here to trace the courses of three series, although two of these are confluent m the leg, the component systems of neurones in terms of Splaai where also the radial marginal group of muscles is absen . the parts of the cord there described: (1) cord. An important difference is to be noted between the muscles Around the dendrites of the large cells of the of the hand and foot in the position of the axis of move- anterior horn are the end-tufts of axons, which can be ment of the digits. In the hand the short muscles are I traced up to the medulla oblongata. These collectively arranged so as to adduct to, or to abduct from, an axis
