Volume 1

Human anatomy : including structure and development and practical considerations / by Thomas Dwight ... [et.al.] ; edited by George A. Piersol.

Date:
[1918], ©1918
Catalogue details

Licence: Public Domain Mark

Credit: Human anatomy : including structure and development and practical considerations / by Thomas Dwight ... [et.al.] ; edited by George A. Piersol. Source: Wellcome Collection.

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    of a primitive axis, the zofochord, takes place among the early formative processes of the embryo. In addition to the fundamental longitudinal axis, vertebrate animals exhibit a transverse cleavage into somatic or body-segments. While such segmentation is rep- resented in the maturer conditions by the series of vertebrze and the associated ribs, the tendency to this division of the body is most marked in the early embryo, in which the formation of body-segments, the som7tes, takes place as one of the primary developmental processes. Although these primary segments do not directly corre- spond to the permanent vertebrz, they are actively concerned in the formation of the latter as well as the segmental masses of the earliest muscular tissue. In man not only the skeleton, but likewise the muscular, vascular, and nervous systems are affected by this segmentation, the effects of which, however are most evident in the structure of the walls of the thoracic portion of the body-cavity. Disregarding the many variations in the details of arrangement brought about by specialization and adaptation, the body of every vertebrate animal exhibits a fundamental plan of construction in which J4zlateral symmetry is a conspicuous fea- ture. Viewed in a transverse section passing through the trunk, the animal body Neural arch Neural tube Spinal cord Costal segment Parietal mesoblast Aorta Parietal mesothelium Visceral mesothelium Entoblastic epithelium Subepithelia] mesoblast Visceral mesoblast Diagrammatic plan of vertebrate body in transverse section. (Modified from Wiedersheim.) may be regarded as composed primarily of the axis, formed by the bodies of the vertebree, and two tubular cavities of very unequal size enclosed by the tissues con- stituting the body-walls and invested externally by the integument (Fig. 1). The smaller of these, the neural tube, is situated dorsally, and is formed by the series of the vertebral arches and associated ligaments ; it surrounds and protects the great cerebro-spinal axis composed of the spinal cord and the specialized cephalic extremity, the brain. The larger space, the visceral tube corresponding to the body- cavity, or celom, lies on the ventral side of the axis and contains the thoracic and abdominal viscera, including the more or less convoluted digestive-tube with its accessory glandular organs, the liver and the pancreas, and the appended respiratory tract, together with the genito-urinary organs and the vascular and lymphatic appa- ratus. The digestive-tube, which begins anteriorly at the oral orifice and opens posteriorly by the anus, is extended by two ventral evaginations giving rise to the respiratory tract and the liver, a dorsal glandular outgrowth representing the pan- creas. The sexual and urinary glands and their ducts primarily occupy the dorsal wall of the body cavity. The vascular system consists essentially of the ventrally placed contracting dilatation, the heart, divided into a venous and an arterial com-