HUMAN ANATOMY. 



of a primitive axis, the notoc/iord, takes place among the early formative processes of 

 the embryo. 



In addition to the fundamental longitudinal axis, \-ertebrate animals exhibit a 

 transverse cleavage into somatic or body-segments. While such segmentation is rep- 

 resented in the maturer conditions by the series of vertebrae 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 somites, takes place as one of the primary 

 developmental processes. Although these primary segments do not directly corre- 

 spond to the permanent vertebrae, 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, hov/ever 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 bilateral symmetry is a conspicuous fea- 

 ture. Viewed in a transverse section passing through the trunk, the animal body 



Fig. I. 



Neural arch 



Neural tube 

 Spinal cord 



Vertebral axis 



Epidermis 

 Corium 



Parietal mesoblast 



Costal segment 

 Parietal mesoblast 



Aorta 



Parietal mesothelium 

 Visceral mesothelium 



Entoblastic epithelium 



Subepithelial mesoblast 

 Visceral mesoblast 



Diagrammatic plan of vertebrate body in transverse section. {Modified from IViedersheim.) 



may be regarded as composed primarily of the axis, formed by the bodies of the 

 vertebrae, and two tubular cavities of very unequal size enclosed by the tissues con- 

 stituting the body-walls and invested externally by the integument (Fig. i). 



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 ccelom, 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- 



