THE NER\^OUS SYSTEM 



71 



its own mechanical axis only, as when it is in an approximately 

 vertical position supporting the weight of the body at rest. When 

 acting as a lever, it exerts both against the body and against the 

 ground forces at right angles to its mechanical axis. 



As a propulsive mechanism, the limb functions in both these 

 wa3^s. As a propulsive strut, it is extended by its own intrinsic 

 musculature and its action may be compared roughly to that of a 

 pole propellmg a punt. As a lever, the limb is operated by the 

 muscles which attach it to the body and its effect is somewhat 

 analogous to that of a double paddle propelling a canoe while the 

 outer end of its blade remains fixed. 



The Nervous System 



There is probably no other system of organs in which external 

 form is so little suggestive of actual function as in the nervous 

 system. This is perhaps less true of its peripheral portion, con- 



FiG. 39. Plan of the central and peripheral connections of a spinal 

 nerve: an, afiferent (sensory) neuron; avn, afferent visceral neuron; ca, cp, 

 anterior (ventral) and posterior (dorsal) columns of gray matter; en, efferent 

 (motor) neuron; esn, visceral efferent (preganglionic sympathetic) neuron; 

 grp, dorsal root ganglion; i, intestine (visceral organs); m, skeletal muscle; 

 na, np, anterior (ventral) and posterior (dorsal) rami of spinal nerve; pg, 

 postganglionic sympathetic neuron; ra, rp, anterior and posterior roots of 

 spinal nerve; rg, rw, gray and white rami communicantes (sympathetic); 

 sk, skin; sp, white matter of spinal cord; ts, ganglion of sympathetic trunk; 

 vm, smooth muscle. Modified, from Herrick. 



