STRUCTURE OF THE HUMAN BODY 



marrow are continuous with one another. One can scarcely say- 

 that the ventral cavity is represented at all in the neck or head. 



N.B. — In the following account of the human body it is 

 supposed to be placed with the front (ventral) surface directed 

 downwards and the back (dorsal) surface directed upwards, as in 

 a quadruped. The "upper" and "lower" parts, commonly so 

 called, will be termed anterior and posterior 

 respectively, unless the ordinary words 

 happen to be specially convenient, as e.g. 

 when the arrangement for supporting the 

 weight of the body are described. 



The body of a large and complicated 

 animal such as a human being requires hard 

 parts to serve as a protection to the delicate 

 internal organs, to act as a stiffening or sup- 

 port, and to furnish points of attachment to 

 the bands or masses of flesh (muscles) by 

 means of which movements are effected. 

 Such hard parts constitute the skeleton, 

 which may be either external {exoskeleton) 

 or internal {endoskeleton), both being present 

 in many animals. In Man the exoskeleton 

 is not so well developed as in many other 

 cases, but it is present in the form of the 

 horny outer layer of the skin (epidermis), 

 and the hairs and nails which grow out from 

 this. Without epidermis the numerous blood- 

 vessels in the deeper part of the skin (dermis) would constantly 

 be wounded, while the many nerves there ending would be the 

 source of constant pain. Any bald-headed person can testify 

 to the value of hair as a protection from extremes of tem- 

 perature and damp, while nails help to protect the sensitive ends 

 of the fingers and toes, though this, of course, is not their only or 

 even their chief use. 



Fig. 3. — Trunk in Longitudinal 

 Section. A, Ventral wall of thorax: 

 B, Diaphragm ; c. Ventral wall of 

 abdomen. To right of E and d is 

 seen backbone in section with spinal 

 canal. 



THE ENDOSKELETON. 



The endoskeleton (figs, j,-']) is built up from bone, grisde 

 (cartilage), and tough fibrous material (connective tissue). We 

 may conveniently distinguish between the axial skeleton (support- 

 ing the head, neck, and trunk) and the skeleton of the limbs. The 



