176 



Human Physiology. 



nerves of organic life. The brain and its nerves of sensation and 

 motion, and all its powers of thought and feeling, are built up- 

 and nourished by, and rest upon, the nervous system of organic 

 life, which has its ganglia, little brains, or vital centres near 

 the heart, the stomach, in the pelvis, and along the spinal column, 

 communicating everywhere with the nerves of the cerebro- 

 spinal system. 



The nerves of organic life control all the more important pro- 

 cesses of the bodily life. They preside over the 

 digestion of our food, the formation and circulation 

 of the blood, its purification by the removal of 

 the waste matter of the system, the action of 

 liver, kidneys, and the myriad glands of the intes- 

 tines, skin, and lymphatic system. The nervous 

 systems of insects (Fig. 30) have a close resemblance 

 to the human ganglionic system. 



Prick the skin anywhere with the finest needle, 

 and you touch nerves of sensation. The blood 

 which follows the prick shows that you have pierced 

 blood vessels, and the organic nerves everywhere 

 accompany these, and preside over their functions. 

 It is the same in the whole interior of the body y 

 only that nerves of sensation are not so fully dis- 

 tributed where they are not required; but the en- 

 tire body is a mass or network of nerves, so that if 

 all other matter were destroyed, the nerves would 

 perfectly define the form of every organ. This is 

 true also of the veins and arteries. 



The mode of action of brain and nerves is but 

 little understood. We observe certain phenomena, 

 but do not know how they are accomplished. The 

 action of some force at the end of my finger makes 

 an impression of pain, or roughness, hardness, smoothness, 

 which impression is conveyed, somehow, by the nerves to the 



Fig. 30. 



NERVOUS 



SYSTEM OF 



INSECTS. 



