342 THE MAINTENANCE OF THE INDIVIDUAL 



polation of adjustor neurons. It is a moot question whether or not 

 the simplest type of refiex arc, involving only two neurons, ever occurs 

 in vertebrates. Most of the so-called "refiex actions" of man are 

 usually not isolated from the rest of the nervous system. 



In lower vertebrates, such reflexes as are concerned with locomotion, 

 breathing, swallowing, and escape from danger are automatic spinal 

 cord reflexes. When it comes to forms with complicated and highly 

 developed nervous systems, such as man, many actions become auto- 

 matic, relieving the brain ordinarily of any responsibility concerning 

 them. In this category fall such phenomena as breathing, sneezing, 

 and shivering. Certain actions, namely jerking of the knee, dodging 

 a blow, closing the eyes to keep out foreign particles, are reflexes 

 which may be controlled or inhibited by a conscious effort. Still more 

 complex reflexes are called into action when playing a musical instru- 

 ment, or in walking, talking, swimming, or driving a car. 



Types of Nervous Systems 



One of the fundamental characteristics of protoplasm is irritability. 

 In simple types of animals, like Ameba or the sponges, where co-ordi- 

 nation between parts is not essential, no specialized nervous system ia 

 developed. With the aggregation of cells in higher forms thera 

 arises the necessity of correlating the interaction of component parts 

 and consequently some sort of definite nervous system has been 

 evolved. To be sure, such devices are quite unspecialized compared 

 with the complicated nervous apparatus of a vertebrate, but never- 

 theless they appear to be reasonably effective. 



Neuromotor Mechanisms 



Ameba, although a very simple type of organism, gives evidence of 

 being definitely affected by stimuli. This is shown by the passage of 

 stimuli from one point on the surface to the general mass of the body, 

 causing the animal to move away from the source of stimulation 

 and resulting in the formation of pseudopodia on the opposite side. 

 Experiments upon Ameba suggest that stimuli are transmitted in the 

 clear outer layer of ectoplasm. 



Probably the highest development of a co-ordinating system among 

 the protozoa appears in some of the ciliates. We have already 

 discussed movement in Stylonychia and in Euplotes. Considerable 



