REFLEX ACTION 589 



and is executed volitionally. The removal of the cerebrum converts this previously 

 complex reaction into a pure reflex, as may be gathered from the fact that the 

 decerebrated frog produces this sound at any time in consequence of such cuta- 

 neous stimulations as the stroking of the skin of the dorsum or the application of a 

 gentle pressure to the sides of the abdomen. Moreover, this reflex may be repeated 

 almost any number of times until reflex-fatigue causes it to cease. Another ex- 

 periment illustrating cerebral inhibition of reflexes, is the following: When the 

 female frog deposits its* eggs, the male endeavors as a rule to aid its mate by 

 firmly clasping her abdomen with his fore limbs. This reaction on the part of the 

 male may be converted into a reflex by removing the cerebrum, as is evinced by the 

 fact that the decerebrated male rtiay be made to clasp objects of any kind by sim- 

 ply bringing them in contact with the ventral aspect of his thorax. In fact, it is 

 possible to produce this reflex even in the absence of all parts excepting the thorax 

 ,and the two forelimbs. In the higher animals, the removal of the cerebrum 

 distinctly shortens the time of the spinal reflexes and leads to the appearance of 

 certain reflexes which under normal conditions are scarcely perceptible. Such 

 acts as licking, scratching, growling, etc., then assume a clear reflex character, 

 because the influence of volition has been permanently removed from them. 



Inhibition by the Midbrain. — It has been assumed that reflex action is regulated 

 by a higher center which, in accordance with Setschenow,i is located in the mid-, 

 brain, i.e., in the optic lobes of such animals as the amphibia and reptilia. This 

 conclusion is based upon the observation that the removal of this part of the 

 nervous system shortens the time of the spinal reflexes and renders them more 

 vivid. The opposite effect may be produced by stimulating these bodies while 

 eliciting any one of the spinal reflexes. The evidence, however, seems to be against 

 the existence of specific inhibitory centers for reflex action. Instead, it is generally 

 assumed that the optic lobes (corpora quadrigemina) and other bodies, are 

 enabled to unfold this faculty in consequence of their connection with the chief 

 conducting channels passing to and from the cerebral hemispheres. In the lower 

 vertebrates, they are of even greater importance, because they give origin to the 

 optic nerves. It is only natural to suppose that the sensory impressions derived 

 from this source must tend to hinder simple reflex action even in the absence of 

 special inhibitory centers. It seems, therefore, that this form of inhibition may be 

 most easily explained upon the basis of a central interference of different afferent 

 impulses with one another. 



Inhibition by Other Afferent Impulses. — It is a well recognized fact that reflexes 

 may be inhibited by simultaneous afferent impulses. The act of sneezing may be 

 suppressed by exerting a gentle pressure upon the upper lip or by rubbing the nose. 

 Quite similarly, a mechanical stimulus to the skin may be rendered abortive by a 

 second stimulus applied elsewhere to the integument. Thus, it may easily be 

 shown that the reflex caused by stimulating the sole of the frog's foot, may be com- 

 pletely inhibited by the simultaneous excitation of the central end of the opposite 

 sciatic nerve. In the absence of distinct inhibitory reflex centers and nerves, these 

 results can only be explained upon the basis of an interference of impulses, result- 

 ing, as has been more fully discussed above, in the ganglion cells of the reflex cir- 

 cuit involved in this particular act. In consequence of the refraction of the cell, 

 one of these impulses is rendered ineffective. 



Strong and continued stimulation of sensory nerves eventually leads to a 

 depression and complete abolition of almost all reflexes. This condition is known 

 as "shock," and if the immediate cause of this depression is located in the realm 

 of the spinal cord, as spinal shock. 



Shock.^ — A person in shock is usually found in a state of complete muscular 



^ Physiol. Studien iiber die Hemmungsmechanismen, etc., Berlin, 1863. Meltzer, 

 The r61e of inhibition in normal and pathological phenomena of life, Med. Record, 

 1902. 



2 Short, Lancet, 1914, and Wiggers, Am. Jour. Med. Sciences, clii, 1917, 666. 



