542 GENERAL ANATOMY OF THE NERVOUS SYSTEM. 



CHAPTER XVI. 



GENERAL ANATOMY OF THE NERVOUS SYSTEM. 



IN man, and animals nearest him in the scale of organi- 

 zation, the nervous system, by its bulk and general distribution, 

 no less than by the important functions it executes, forms a 

 very prominent part in the constitution of the body. So early 

 as the time of Herophilus, who flourished nearly three centuries 

 before the commencement of the Christian era, the connexion 

 of the nerves with the brain had been discovered ; as well as 

 the fact that these parts were to be regarded as the instruments 

 of sensation and motion. In the sixth century, the idea was 

 also entertained by Galen that there was some specific differ- 

 ences among the nerves. He held that the nerves of sensation 

 originated in the brain, and those of motion, in the spinal 

 cord. Closely connected, as it was found to be, with the phe- 

 nomena of life, the nervous system has been, since the com- 

 mencement of the seventeenth century, a subject of much re- 

 search among anatomists; but, from its being the most delicate 

 and complicated in its structure of all the tissues of the body, it 

 has necessarily required extremely careful and oft repeated ex- 

 amination, to arrive at any accuracy in regard to its structure. 

 The progress made in its investigation has consequently been 

 slow. Comparative anatomy has lent its aid to the elucidation 

 of nervous structure. One after another, in ttye downward grade 

 of animals, have been found provided with nervous centres and 

 nervous cords, and the recent observations of Tiedemann, Ehren- 

 berg, Remak, etc., have rendered it extremely doubtful that any 

 animal exists, without a nervous system properly proportioned 

 to the force and vitality of its vital actions. 

 The nervous structure of man and many of the inferior 

 animals, is found disposed in masses, as in the brain and spinal 

 marrow; rolled up in the form of round cords, called nerves, 

 which branch in every direction through the body ; and as 

 small knots, or ganglia, which are developed in the course, or at 



