OF THE BLOOD AND ITS VESSELS. 327 



where there is sensation or feeling, nerves are supposed 

 to be present. The functions of the nerves, producing 

 muscular motion, are sometimes independent of the will, 

 as in the beating of the heart, and in others obedient 

 thereto, as in the action of the locomotive muscles, ex- 

 ercised in walking, or in the use of the arms and hands. 

 That the action of the muscles, as well .as every spe- 

 cies of sensation, depends on the nerves, is proved by 

 the fact that all such phenomena cease, when the nerve 

 supplying any particular part is separated. Thus, when 

 the nerve which passes to the arm is cut, that member 

 no longer obeys the will ; and the sight is instantly 

 destroyed if the optic nerve is severed ; and so of every 

 other portion of the system. The nervous trunks, which 

 issue in pairs from the brain and spinal cord, amount to 

 about 40 in number; and in tracing them and their 

 branches, they are found in certain parts of the system 

 to swell into knots called ganglia, a figure and descrip- 

 tion of which will be seen at page 158. 



OF THE BLOOD AND ITS VESSELS. 



Having thus shown how the bones are connected and 

 put into motion, and from what source their motion is 

 derived, we- will next point out the means by which the 

 muscles, as well as other parts of the living system, 

 grow, and are nourished. 



The circulation of the blood, together with the extra- 

 ordinary effects which respiration has on that fluid, have 

 already been described severally at pages 127 and 163. 

 Without this exposure to the air in the lungs, the blood 

 is unfit for the purposes of life, nor can any animal exist 

 more than a few minutes without it, strangulation being 

 the speedy consequence. The heart is therefore so con- 

 structed as to propel the blood which it receives through" 

 the substance of the lungs by means of the arteries, 

 which are there intermixed with the air-vessels, and thus 

 the blood undergoes, by the influence of the air we take 



Is muscular motion always dependent on the will ? How is it proved that 

 sensation depends on the nerves ? How many pairs of nerves are there ? 

 What are ganglia? 



