THE NERVOUS SYSTEM. 231 



10. Write a description of your dissection of the nerves of the thoracic 

 limb. 



11. Mention in what way the lumbar plexus differs from the descrip- 

 tion in the book. 



12. Draw the great sciatic nerve and its branches as seen from the 

 caudal aspect. 



13. What nerves supply the digits? 



14. Describe the chief plexuses of the sympathetic system. 



15. How do stimuli from the viscera reach the brain? 



16. What portion of the body is not supplied with nerves from the 

 sympathetic system? 



THE ORGANS OF SENSE. 



The organs of sense are the specialized peripheral 

 terminations of the sensory nerves, and are so constructed 

 as to be capable of receiving only a certain kind of 

 stimulus. The stimuli for the eye are ether vibrations; 

 those for the ear are vibrations of the air. The stimu- 

 lation of the sensory nerves produces sensations in the 

 cells of the cerebral cortex to which they lead. 



The external stimuli giving rise to the internal sen- 

 sations of seeing, hearing, smelling, and tasting are 

 transmitted by only four pairs of cranial nerves, while 

 the stimuli of cutaneous sensations are transmitted 

 by three pairs of cranial nerves and all the spinal nerves. 



Cutaneous Sense Organs.- -The cutaneous sense organs 

 are composed of the endings of the sensory nerves in 

 all parts of the skin and the mucous membrane of the 

 mouth, nose, arms, vagina, and urethra. One kind of 

 sense organs, those of pain, are present in every organ of 

 the body. The sense organ of pain is probably an 

 unmodified free nerve-ending. 



While all portions of the skin and perhaps other 

 parts of the body are supplied with organs capable of 

 receiving stimuli giving rise to tactile sensation, the 

 soles of the feet and the skin at the base of the vibrissae 



