600 NERVOUS SYSTEM. 



particularly frogs, co-ordinate reflex movements are often observed, such as 

 the movements of jumping or swimming. This is sometimes called purposive 

 reflex action, as the movements seem to have a definite purpose or object. The 

 following well known experiment illustrates a co-ordinate, or purposive reflex : 



Pfliiger (1853) removed the entire encephalon from a frog, leaving only 

 the spinal cord. He then touched the surface of the thigh, over the inner 

 condyle, with acetic acid. The animal thereupon rubbed the irritated sur- 

 face with the foot of the same side, apparently appreciating the seat of the 

 irritation, and endeavoring, by a voluntary effort, to remove it. The foot of 

 this side was then amputated, and the irritation was renewed in the same 

 place. The animal made an ineffectual effort to reach the spot with the 

 amputated member, and failing in this, after some general movements of 

 the limbs, rubbed the spot with the foot of the opposite side. 



It has been thought that this experiment shows a persistence of sensa- 

 tion and the power of voluntary movements after removal of the entire en- 

 cephalon ; but it must be remembered that the cord contains cells connected 

 together by fibres probably into groups which correspond to sets of muscles 

 concerned in co-ordinate movements, and that many movements set in action 

 by an effort of the will continue in an automatic manner, as the ordinary 

 movements of progression. It is more reasonable to suppose that a persist- 

 ent stimulation of the surface, such as is produced by the action of acetic 

 acid upon the skin of a frog, can give rise to co-ordinate movements of a 

 purely reflex character than to assume that the movements in Pfliiger's ex- 

 periment are voluntary efforts to remove a painful impression. It is certain 

 that in the higher classes of animals after removal of the encephalon, in ex- 

 periments on decapitated criminals and in patients suffering from paraplegia, 

 there is no evidence of true sensation or volition in the spinal cord. In man 

 and the higher animals, all muscular movements which depend solely upon 

 the reflex action of the cord must be regarded as automatic and entirely in- 

 dependent of consciousness and of the will. 



Certain reflex movements may be restrained by an effort of the will, as is 

 well known ; provided, always, that these be movements that can be exe- 

 cuted by voluntary effort. Nevertheless, if the sensory impression be suffi- 

 ciently powerful or be very frequently repeated, it is often impossible to con- 

 trol such movements by the will. Movements that are never in themselves 

 voluntary, such as the ejaculation of semen, when excited by reflex action 

 can not be restrained by a voluntary effort ; while the reflex act of coughing, 

 for example, may be measurably controlled. It is hardly proper to speak of 

 inhibition of the reflexes, in the sense in which the term inhibition is gener- 

 ally used in physiology, for the reason that there are probably no special in- 

 hibitory nerves for these movements. 



Various reflexes are made use of in pathology as means of diagnosis. 

 The superficial reflexes are those produced by tickling the soles of the feet 

 or by exciting other parts of the skin. The most prominent of the deep re- 

 flexes is the patellar reflex, or the knee-jerk, produced by percussion of the 

 ligamentum patellae. 



