192 ELEMENTARY PHYSIOLOGY 



QUESTIONS ON LESSON XXXII. 



1. Describe the structure of the spinal cord as revealed in a transverse sec- 



tion. Illustrate your description by a sketch. 



2. How is the spinal cord protected ? Compare it in this respect with the 



brain. 



3. How are the spinal nerves arranged ' How do they pass out from the 



spinal canal ? 



4. Describe the arrangement and the functions of the roots of the spinal 



nerves. 



5. How would you prove (i) that the anterior roots consist of motor fibres 



only, (2) that the posterior roots consist of sensory fibres only, and (3) 

 that the spinal nerves contain both sensory and motor fibres ? 



6. Give a proof that the spinal cord is a nerve centre. 



7. How is it that a man in a fit, though quite unconscious, may go on 



breathing regularly ? What part of the nervous system keeps up and 

 regulates the act of breathing ? 



8. Why is a man whose back is broken unable of himself to move his legs, 



though they may move involuntarily when the feet are tickled ? 



9. When a man's neck is broken death is sometimes instantaneous. How 



is this brought about ? 



10. How do we know that the nerve fibres which are employed when a limb 



is moved are different from those which are set in action when pain is 

 felt in that limb ? 



1 1. When your leg is kept for a long time in one position it sometimes ' goes 



to sleep.' What is the condition of the limb at this time, and how is 

 it brought about ? 



12. Instances of injury or disease have been met with in which the power of 



movement in a limb has been lost and yet feeling has remained, and 

 others in which feeling has been lost and yet the power of movement 

 has been retained. State what parts must have been affected by the 

 injury or disease in these cases. 



LESSON XXXIII. 



SENS A TIONS TO UCH. 



THE nervous system is the medium by which we derive a know- 

 ledge of the existence of the various parts of the body, and of the 

 external world. This knowledge is based upon sensations. 



In _ order to produce a sensation of any kind, three distinct 

 conditions are necessary. These are: (i) some disturbing con- 

 dition which irritates one or more sensory fibres ; (2) the trans- 

 mission of this irritation along the sensory fibres to the nerve-centre 

 (the brain); and (3) the translation of the irritation in the nerve- 

 centre into a state of consciousness. 



Some of the sensations, such as sight and hearing, are produced 

 by means of a specially constructed organ of sense, so that they 



