110 UNIVERSAL EVOLUTION 



parts of the brain structure, centering in the cerebrum, 

 by the cross association fibres. 



REFLEX ARCS. The unit of mechanism, in the ner- 

 vous system, is the reflex. The simplest reflex arc con- 

 sists of a receptive nerve running from a receptive organ, 

 the eye, for instance, to the appropriate brain center 

 of such organ, and from there to the appropriate 

 muscle. In animals of very low structure the nerve 

 current is made directly over a single reflex arc and 

 the reaction follows the sensation immediately. But in 

 animals of complex structure, having large ganglia called 

 brains, the reflex arcs are compounded and multiplied. 

 In man, this complexity is capable of controlling the ac- 

 tion of the sensation, so that there is deliberation, hesita- 

 tion, and calculation, before any muscle movement is 

 made. A characteristic of the system is, that the chain 

 of nerves forming the reflexes are joined together, in 

 such a way, that the sensations received at the peri- 

 pheral, can be conducted only toward the ganglion to 

 which the nerve runs. This receptive nerve, however, 

 receives impulses generated in various places in the 

 body, and from other nerves near it. In this way, a 

 muscle can receive impulses from many reflex arcs. In 

 this way, a stimulation received through any one sense 

 organ arouses the expectation of all the muscles of the 

 body, if it happen to be of such nature as to demand 

 it. It is thus that a soldier marching toward a field 

 of battle, and hearing the boom of artillery, ahead of 

 him. has all his senses aroused through the single sense 

 of hearing. The muscles of his whole body are excited, 

 by the expectancy of battle. A limb muscle is the 

 terminus of nervous arcs arising in everyone of the 

 organs of sense, the eye. the ear, the nose, the skin, 

 and the mouth. In the works of Edinger and Exner 



