290 TMi: INTHRNAL ENVIRONMENT OF THE BODY Part III 



blood. This, like other true rellexes, is an experience common to all indi- 

 viduals of one species or to many species within a large group. A young 

 mammal sucks milk whether it is cat, whale, or human. Other pure reflexes 

 arc the quick closure of the eyelid when something comes near the eye; the 

 sudden pullback of the hand that is pinched; the sharp recovery of balance 

 lost in a stumble. 



Conditioned responses, formerly called conditioned reflexes, were demon- 

 strated by Ivan Pavlov (1849-1936), a Russian physiologist. Over and over 

 he attracted a dog's attention by the sound of a bell, then gave it food, and 

 observed the flow of its saliva. After being fed many times at the ringing of 

 the bell, then the bell was rung but no food was offered. In such cases, the 

 bell alone stimulated a flow of saliva from the dog's mouth. Pavlov christened 

 the response a conditioned reflex but the better name, conditioned response, 

 is taking its place. It is habit formation like eating, sleeping, and waking at 

 definite times. 



The Functional Unit — the Reflex Arc or Reflex Response. The action of 

 a great number of reflexes never goes higher than various levels of the spinal 

 cord, never enters the brain at all. Many that seem simple actually involve 

 many nerves and muscles and are very complex. One may touch a hot iron 

 with one's hand and pull it away, skew the body suddenly, and step back. 

 The action is all reflex of which the brain is notified only by means of asso- 

 ciated nerve cells. One of the simplest of human reflexes is the knee-jerk. 

 This is well known, not only as an example of a simple reflex, but as the one 

 used in routine tests of nervous adjustments. A slight blow on the kneecap 

 (actually on the patellar ligament) when the legs are crossed will normally 

 cause the foot to jerk forward. The jerk will not occur if the sensory roots of 

 the spinal nerves are damaged, as in locomotor ataxia (tabes dorsalis) or 

 if the gray matter of the cord is damaged, as in infantile paralysis. 



The basic unit of function called the reflex arc is typically carried out 

 by five parts: (1) a sensory receptor cell; (2) a conductor, the sensory 

 nerve cell; (3) a connecting or adjuster nerve cell (in the cord); (4) a con- 

 ductor, the motor nerve cell; and (5) an effector, muscle or gland cell 

 (Figs. 16.9, 16.11). 



Actual Conditions of the Reflex Response. The usual diagram of a reflex 

 arc shows a single sensory nerve cell by which an impulse is transmitted 

 directly to a single motor cell or with an adjustor cell between them. Actually, 

 in all vertebrate animals, the simplest stimulus starts impulses through several 

 sensory fibers with a volley of them following one another in quick succession 

 along each fiber. And each sensory cell fiber is not in contact with only one 

 adjustor or one motor cell but with several of them. It is only when impulses 

 arrive at almost the same time via a number of sensory fibers that the motor 

 nerve cells are finally activated. A certain degree of stimulation (summation) 



