STRUCTURE OF THE HUMAN MECHANISM 27 



19. The capillaries. The blood which enters an organ 

 through the arteries passes to its veins through a system of 

 microscopic tubes (Fig. 16), the capillaries (Latin capilla, " a 

 hair") ; these may be readily seen under the microscope in 

 the web of a frog's foot. From the foregoing description of 

 the course of the circulation it will be observed that gen- 

 erally the blood must pass through one set of capillaries in 

 going from the aorta to the vena3 cavse or from the pul- 

 monary artery to the pulmonary vein ; but the blood which 

 flows through the capillaries of most of the abdominal organs 

 (stomach, intestines, spleen) must pass also through a second 

 set of capillaries, namely, those of the liver, before it can 

 return to the heart. 



20. Organs of the nervous system. The skull and the 

 spinal column (p. 18) are chiefly occupied by the brain and 

 the spinal cord, respectively, and from each of these principal 

 organs of the nervous system branches consisting of cords 

 of nervous substance, the nerves, pass out through small 

 holes in the skull or spinal column and are distributed to 

 all the other organs, where they terminate in peculiar struc- 

 tures called end organs. The optic nerve, for example, ends 

 in the retina, the auditory nerve in the inner ear, and motor 

 nerves in muscles the nerve endings in these different 

 organs differing materially in structure and arrangement. 



Fig. 17 gives some idea of the general arrangement of 

 the nervous system. The nerves to the shoulder, arm, and 

 hand will be seen to arise from the cervical region of the 

 spinal cord ; those for the trunk, from the dorsal and lumbar 

 regions ; those for the legs, from the sacral region. The head 

 and face receive nerves from the posterior portions of the 

 brain. The dissection of the arm in Fig. 18 shows more 

 accurately the main nerve trunks to that region. Further 

 information with regard to the structure of the nervous 

 system will be given in Chapters VII, XIV, and XV. 



