6 THE HUMAN BODY 



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

 nose, i, and mouth, o; h is the heart, I a lung, s the sympathetic 

 nervous system, and k a kidney. 



A transverse section through the chest is represented by the 

 diagram Fig. 3, where x is the neural canal containing the spinal 



FIG. 3. Cross-section of thorax. A, bronchus, entering the lung; B, the aorta 

 cut at its origin and again at the descending part of its arch; C, the pericardial 

 space; D, the pleural cavity; E, the alimentary canal; PA, the pulmonary artery; 

 X, the neural canal. 



cord. In the thoracic cavity are seen the heart, the lungs, part of 

 the alimentary canal, E; bronchial tubes, A, leading to the lungs; 

 and blood vessels, B and P A, communicating with the heart; 

 the heavy line on each side covering the inside of the chest-wall and 

 the outside of the lung represents the pleura. 



Sections through corresponding parts of any other Mammal 

 would agree in all essential points with those represented 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 is enveloped in skin. The 

 only cavities in the limbs are branching tubes which are filled with 



