840 THE CENTRAL NERVOUS SYSTEM 



ma l bulbospinal, as it is called shows a greater integration of re- 

 flexes than is possible when the section is between the medulla and the 

 spinal cord. Its reflex responses are more broadly integrated, but the 

 extremities are incapable of executing movements that are of any value 

 in progression. Movements like those of progression may occur, but they 

 are quite ineffective. Such animals show marked superiority over strictly 

 spinal ones on account of the fact that in the medulla are located so 

 many of the important centers which control circulation, respiration 

 and the anterior openings of the body; that is, the mechanisms which 

 accompany the first stages in the digestion of food. 



Section Just Behind the Posterior Corpora Quadrigemina. A very 

 distinct improvement becomes noticeable in the responses of the animal. 

 This condition has been studied most carefully in the case of the frog, 

 which after such a section can walk, spring and swim apparently like a 

 normal animal, and croaks when the side of the body is stroked. In 

 the mammal a similar increase in the complexity of movement is evident, 

 but there is not yet any power of progression. 



Section in Front of the Anterior Corpora Quadrigemina. When the 

 medulla, pons and mesencephalon are present, as well as the spinal cord, 

 the condition known as decerebrate rigidity supervenes. This is most 

 marked in mammals, but is also present to a certain extent in much 

 lower animals, as, for example, in frogs. It consists of a tonic condition 

 of the postural musculature of the body, mainly of the extensor mus- 

 cles; the elbows and knees are extended and they resist passive flexing; 

 the tail is stiff and straight ; the neck and head are retracted. The con- 

 dition is undoubtedly due to overactivity of the reflex tonic function of 

 the spinal centers, for it disappears when the posterior spinal roots are 

 cut. The reflexes that depend on the tone of the musculature for example, 

 the knee-jerk and extensor thrust are very pronounced in such an ani- 

 mal, and, on account of the higher integration present, reflexes appear 

 that are absent in animals having the cerebrospinal axis, cut lower down. 

 For example, although such an animal can not feel, yet when a stimulus 

 is applied that in a normal animal would cause pain, the vocal apparatus 

 may be excited so that a sound or cry of pain is produced. The rigidity 

 does not affect the respiratory muscles. After such an operation, how- 

 ever, normal respiration is much more likely to be maintained if the 

 section is in front of the anterior corpora quadrigemina than behind it. 



Removal of the Cerebral Hemispheres. This furnishes us with what 

 is known as a decerebrate preparation that is, one in which the animal 

 retains everythirig from the basal ganglia downward. The operation 

 produces a condition which varies according to the habits of the animal. 

 Thus, in such fish as the Elasmobranchs, which depend for their impressions 



