ANIMAL FUNCTIONS. 97 



functions. On the other hand any increase in the supply of a part 

 is attended with an exaltation of its function, as is seen in active con- 

 gestioti of the brain and spinal cord. It is now a generally received 

 physiological truth, that the functional activity of the nervous system 

 is mainly dependent, not only upon the due supply of arterial blood, 

 but also upon the combination of its oxygen with the elements of the 

 nervous structure. 



For physiological consideration the nervous system may be divided 

 into three great divisions, the cerehro-spiiial ; the reflex^ or true 

 spinal ; and the great sijinpoxhetic , 



The cerebrospinal includes the cerebrum and cerebellum, with 

 the sentient and motor nerves, that run to and proceed from them 

 along the base of the brain, or along the spinal marrow to every part 

 of the system. It presides over sensation and voluntary motion. 



The reflex, or true spinal, includes ihe gray matter of the medulla 

 oblongata ^nA spinalis as its ce^^^/•e, and a peculiar set of fibres 

 running to and proceeding from these centres, called afferent, or 

 excitor, and efferent, or 7notor. It presides over involuntary or ex- 

 cited movements. 



The great symjmthetic, or ganglionic, consists of a series of gan- 

 glia on each side of the vertebral column, extending from the base 

 of the cranium to the os coccygis, and communicating both with the 

 spinal and encephalic nerves, sending its branches along the arteries, 

 and particularly to the organs of involuntary functions. Its office is 

 supposed to be to bring the functions of organic life into relation 

 or sympathy with those of animal life. 



The changes which take place in a nerve of any of the above di- 

 visions, when it is in action, are known to us only by the effects they 

 produce on the sentient mind, or on muscular parts. There is no 

 alteration in the physical appearance of the nerve or its fibres, which 

 can be detected by our aided or unaided vision, and yet, from the in- 

 stantaneous effect produced by stimuli, and its as sudden cessation 

 on their withdrawal, we can refer the phenomena to nothing so 

 readily as to a molecular change, rapidly propagated along the course 

 of a nerve from the point of application of the stimulus. According 

 to Bowman, a state of polarity is induced in the particles of the 

 nerve by the action of the stimulus, which is capable of exciting an 

 analogous change in other particles, whether muscular or nervous ; 

 whence results the peculiar effects of the nerve's influence. If this 

 doctrine be tenable, the inference results that the nerves are not mere 

 passive conductors, but that the whole extent of the fibre between the 

 stimulated point and the peripheral extremity, or central termination, 

 is the seat of change. 



The organic changes produced in a nerve by either mental or 

 physical stimuli, develope that remarkable power known as the ner- 

 vous force, or vis nervosa. Of the nature of this power we know 



9 



