THE NERVOUS SYSTEM—GENERAL CONSIDERATIONS. 913 
the frog hang in the liquid; after a few moments it will be 
withdrawn. 4. Repeat, holding the leg; probably the other leg 
will be drawn up. 5. Apply stronger acid to the inside of the 
right thigh; the whole frog may be convulsed, or the left leg 
may be put in action after the right. Even if the stimulating 
paper be applied near the anus, it will be removed by the hind- 
legs. 6. Beneath the skin of the back (posterior lymph-sac) 
inject a few drops of liquor strychniw of the pharmacopoeia ; 
after a few minutes apply the same sort of stimulus to the 
thigh as before. The effects follow more quickly and are 
BRAIN ABOVE MEDULLA 
SENSORY CENTRE™~"\----- 77-77 J--7 INHIBITORY CENTRE 
Pee Tart CELL AND 
REFLEX__|. AFFERENT NERVE 
CENTRE 
MOTOR CELL AND 
EFFERENT NERVE 
' 
AUTOMATIC CENTRE: HEE MOTOR CELL WITH 
EFFERENT NERVE 
Fia. 195.—Diagram intended to illustrate nervous mechanism of—1, automatism ; 2, reflex 
action ; and 3, how nervous impulses in the latter case may pass into the higher parts of 
brain and become part of consciousness, or be wholly inhibited. A reflex or automatic 
center may for the sake of simplicity be reduced to a single cell, as above on the left. 
much more marked—the animal, it may be, passing intoa gen- 
eral tetanic spasm. 
These experiments may be varied, but suffice to establish 
the following conclusions: 1. The stimulus is not immediate- 
ly effective, but requires to act for a certain variable period, 
depending chiefly on the condition of the central nervous sys- 
tem. 2. The movements of the muscles harmonize (are co-ordi- 
nated), and tend to accomplish some end—are purposive. If 
