10 THE HUMAN BODY. 



sented in the figure. The thick black mass below the 

 neural tube is part of the spinal column ; bounded by this 

 dorsally, by a rib, r, r, on each side, and by the breastbone, 

 st, on the ventral side (below in the figure) is the haemal 

 cavity, containing the lungs, Z, I ; the heart, h; the gul- 

 let, a; and the sympathetic centers, sy. 



FIG. 3. A section across the forearm a short distance below the elbow-joint. R 

 and U~, its two supporting bones, the radius and ulna; e, the epidermis and d, the 

 dermis, of the fkin; the latter is continuous below with bands of connective tissue, 

 s, which penetrate between and invest the muscles, which are indicated by numbers, 

 ft, ft, nerves and vessels. 



The Limbs. If, instead of the trunk of the body, our 

 section were made across one of the limbs, we should find 

 no such arrangement of cavities on each side of a bony 

 axis. The limbs have a supporting axis made of one or 

 more bones (as seen at U and R, Fig. 3, which represents 

 a section made across the forearm near the elbow joint), 

 but around this axis soft parts, chiefly muscles, are closely 

 packed ; and the whole, like the trunk, is enveloped by 

 skin. The only cavities in the limbs are branching tubes, 

 which are filled during life either with Uood, or a watery- 

 looking liquid known as lympli. These tubes, the Hood 

 and lympli vessels respectively, are not, however, character- 

 istic of the limbs, for they also exist in abundance in head, 

 neck and trunk. 



How do the limbs differ from the trunk? How is each limb 

 supported? Describe the parts exposed on a cross section of the 

 forearm? What cavities exist in a limb? What do they contain? 

 Are they found in other parrs of the body? 



