KEFLEX ACTION 583 



nature of which may be more clearly portrayed by severing the con- 

 nections between it and the brain. This end may be attained 

 by a section made either above or below the medulla oblongata. It 

 will be shown later on that an animal of this kind retains all those 

 functions which are ordinarily accomplished with the aid of the cord. 

 These responses, however, need not remain confined to a particular 

 segment of this structure, but may also involve higher or lower spinal 

 centers without losing their reflex character. The reactions of a 

 ''spinal cord animaP' must necessarily be non-volitional. 



When referring to reflex circuits and actions we are accustomed 

 to associate them immediately with the spinal cord. The preceding 

 discussion, however, must have made it clear that they are not ex- 

 clusively confined to this structure, but may involve almost any part 

 of the nervous system. It seems that the spinal cord is referred to 

 most frequently in this connection, because it is a relatively simple 

 matter to isolate it and to stimulate its nerves. Moreover, the spinal 

 reflexes are perfectly conscript actions and pursue easily recognizable 

 paths. 



CHAPTER XL VIII 

 REFLEX ACTION^ 



The Different Types of EfEectors and Receptors. — If we adhere to 

 the general definition that a reflex is a non-volitional motor response 

 to a sensory impulse, the very diverse and complex character of these 

 reactions must immediately become evident. On the efferent side, 

 of course, conditions are relatively simple, because the effectors consist 

 of only two structural units, namely, the muscle cell and the gland 



1 The term sympathy or consensus was applied by the ancients to almost all 

 phenomena of life. In 1649, however, Descartes separated from these general 

 reactions all those which did not produce an impression in consciousness and were 

 not subjected to the will. He applied to this class of reactions the term reflex, 

 because in analogy to the reflection of light, the sensory impression seemed to be 

 returned in the form of a motor effect. Subsequent to this time, Willis (1664), 

 Astrue (1743), and Unzer (1771) have described various reflexes such as the acts of 

 coughing, sneezing, the closure of the eyelids, the ejaculation of the semen, and 

 others. Their idea, however, seemed to be that these reactions can be brought 

 about with the help of the nerve trunks and do not require a center. Whytt 

 (1751) then proved that this conception is incorrect, because in the frog the 

 destruction of the spinal cord immediately destroys the reflex actions ordinarily 

 elicited with the help of this part of the nervous system. He also described the 

 reflex secretion of the tears and of saliva, and recognized the fact that the latter 

 may also be obtained by psychic stimulation; in other words, he recognized the 

 association reflex. The modern conception of reflex action is based upon the work 

 of M. HaU (1832-33) and Joh. MuUer (1833-34). 



