CHAPTER SEVENTH. 



THE INSTRUMENTS OF ANIMATION.— NEUROLOGY, OR 

 THE BRAIN AND NERVES. 



DEFINITIONS AND DESCRIPTIONS. 



562. Main Features of the Nervous System. — We 

 now come to an organization which is very complicated in 

 structure, some of whose functions are the most obscure of 

 any in the body. It is called the Nervous System : and the 

 different grades in the animal kingdom are established by 

 placing those having the most complicated nervous system 

 highest on the scale. Man having the largest brain in pro- 

 portion to the rest of his body, and possessed of the greatest 

 relative amount of nerves, is therefore placed at the head of 

 the animal kingdom. 



563. Microscopic Structure —Tubular Portion— Diam- 

 eter of the Tubes. — In microscopic structure the nervous 

 tissue presents two essential elements : the fibrous or tubular, 

 which is mainly found in the nerve trunks, and the cellular or 

 vesicular, existing more abundantly in the ganglia or nerve 

 centers. In the former the tubes are the largest in the trunks 

 of the nerves, and gradually diminish as they approach the 

 brain, varying in size from the 2 oV oth to the stIo oth of an 

 inch, and sometimes existing even as large as the ^th of an 

 inch. They are sometimes conical also, measuring at one end 

 from T 2 ! o oth of an inch to 04 £ooth : the smallest end being 

 found near the nerve trunks, and are sometimes called coarse 



562. Give the leading features of the nervous system. 563. What are the two micro- 

 scopic elements of nervous tissue ? Give the size of the tubular portion. 



