116 THE INDIVIDUAL ORGANISM 



ous system would entail a preliminary study of the tremendously intricate 

 details of neural anatomy. This would lie far beyond the scope of our 

 treatment. Moreover, all our present-day knowledge falls far short of 

 describing, to say nothing of explaining, such higher neural functions 

 as those we call memory, consciousness, volition or "willing," and the 

 whole complex that we vaguely describe as "intelligence." What follows 

 can be no more than a summary of some of the simpler aspects of the 

 functioning of the nervous system. 



First-level response — the spinal cord. An animal that has been 

 deprived of its head may be capable for a short time of showing response 

 to stimuli. The physiologist often prepares an experimental animal by 

 removing the part of the brain that has to do with consciousness and 

 "willing." One of the simplest (and most limited) preparations is made 

 by snipping off the head and brain of a frog and leaving the rest of the 

 body intact. In half an hour or so, when the severed nerves in the cord 

 have had time to recover from the "shock," the animal will still show a 

 considerable variety of actions. If one of the toes is pinched, the leg 

 will be lifted and the toe pulled away from the pinch. If a drop of some 

 irritant, a weak acid, for instance, is placed on the animal's chest, it will 

 brush the spot with its hand, and if its arms are now held immovable, 

 it will bring up one of its feet to brush across the spot; if that foot is held, 

 it will often then use the other foot. Such action is carried out without 

 aid from the brain, but it does involve both the nerves and the spinal 

 cord. The term reflex action is applied to responses of this order, and they 

 are carried out by means of the reflex arc. 



The reflex arc. A simple spinal reflex arc is diagramed in Fig. 7.6. 

 Here an outline of a cross section of the spinal cord shows three diagram- 

 matic neurons. The neuron with its cell body located in the dorsal-root 

 ganglion is a sensory, afferent (Latin, "carrying to") neuron, with its 

 exceptionally long dendrite 1 extending to a sense receptor (let us say a 

 pain receptor in this case) in the skin; the axon of this sensory neuron 

 extends into the cord, where it makes a synapse with one of the dendrites 

 of an adjustor neuron, which lies wholly within the cord. The axon of this 

 adjustor neuron makes a synapse with one of the dendrites of the third 

 neuron, a motor, efferent (Latin, "carrying from") neuron in one of the 

 "ventral horns" of the gray matter of the cord. The axon from this 

 neuron extends out from the cord to end in skeletal muscle fibers. Here 

 is one of the simplest nerve sequences that can provide for the reception, 

 transmission, and "delivery" of a nerve impulse. 



1 Unfortunately there is considerable divergence in the definition of many terms 

 that apply to the nervous system (and other body parts) ; this afferent process of the 

 spinal sensory nerve is physiologically and embryologically a true dendrite, although 

 very different in appearance from the dendrites of a motor neuron. 



