CROSS-SECTION OF THE BODY. 7 



them; ee is the vertebral column with its modified prolon- 

 gation into the head beneath the anterior enlargement of 

 the dorsal cavity; / is the alimentary canal opening in front 

 through the nose, i, and mouth, o; li is the heart, I a lung, 

 8 the sympathetic nervous system, and k a kidney. 



A transverse section through the chest is represented 

 diagrammatically in Fig. 3, where x is the neural canal 

 containing the spinal cord. In the thoracic cavity are seen 

 the heart, h, the lungs, II, part of the alimentary canal, a, 

 and the sympathetic nerve centres, sy; the dotted line on 

 each side covering the inside of the chest wall and the 

 outside of the lung represents the pleura. 



FIG. 3. A diagrammatic section across the Body in the chest region, x. the 

 dorsal tube, which contains the spinal cord; the black mass surrounding it is 

 a vertebra; a. the gullet, a part of the alimentary canal; ft, the heart; sy, sym- 

 pathetic nervous system; II, lungs; the dotted lines around them ara the 

 pleurae; rr, ribs; st, the breastbone. 



Sections through corresponding parts of any other Mam- 

 mal would agree in all essential points with those repre- 

 sented in Figs. 2 and 3. 



The Limbs. The limbs present no such arrangement of 

 cavities on each side of a bony axis as is seen in the trunk. 

 They have an axis formed at different parts of one or more 

 bones (as seen at U and R in Fig. 4, which represents 

 a cross-section of the forearm near the elbow joint), but 

 around this are closely-packed soft parts, chiefly muscles, 

 and the whole js enveloped in skin. The only cavities in 

 the limbs are branching tubes which are filled with liquids 

 during life, either blood or a watery-looking fluid known as 

 tywph. These tubes, the Uood and lymph vessels respec- 



