392 PHYSIOLOGY 



connection with the termination of the vagus nerves of this part of the brain 

 is the location here of the chief vaso-motor centre, i.e. in a region which is 

 in close proximity to the endings of the chief afferent nerves from the heart 

 and larger blood-vessels and to the nucleus of the efferent controlling nerve 

 to the heart. 



THE METENCEPHALON (PONS VAROLII AND CEREBELLUM) 



Destruction of the brain at the front of the fourth ventricle and just 

 behind the posterior quadrigemina will leave the animal with a central 

 nervous system, which is in connection by efferent nerves with the whole 

 musculature of the body (with the exception of certain eye muscles) and 

 which receives impressions through the spinal cord from the whole surface 

 of the trunk and limbs, and through the fifth nerve from the face and head, 

 and also the higher specialised impressions from the organ of hearing and 

 the organ of static sense. The impressions from the two great projicient 

 senses of smell and sight would be wanting. 



Such an arnimal presents considerable advance in the complexity of its 

 reactions above one possessing only spinal cord and bulb. The frog, for 

 instance, after such an operation, can still walk, spring, and swim ; when 

 placed on a turntable it reacts to passive rotation by turning its head in the 

 opposite direction. On stroking its back it croaks. If the cerebellum be 

 also removed, the animal becomes spontaneously active and crawls about until 

 it is blocked by some obstacle. In this condition there is great activity of the 

 swallowing reflex. Anything which touches the mouth is snapped at. If 

 placed on its back the frog at once rights itself. 



In the mammal a similar increase of reflex activity is observed though the 

 power of progression is not retained. 



THE MESENCEPHALON OR MID-BRAIN 



A section in front of the anterior corpora quadrigemina would leave 

 the animal with the nervous system receiving all normal sensory impressions, 

 with the exception of the olfactory, and with efferent paths to all the muscles 

 of the body, including those of the eye. In the mammal such an operation 

 brings about a condition known as ' decerebrate rigidity.' Though respira- 

 tory movements continue normally, the whole musculature is in a cataleptic 

 condition, the elbows and knees being extended and resisting passive flexion ; 

 the tail is stiff and straight, the neck and head retracted. This condition 

 seems to depend on an over-activity of the reflex tonic functions of the lower 

 centres. That it is reflex is shown by the fact that the rigidity is at once 

 abolished in a limb on dividing the appropriate posterior roots. The 

 position of the limbs may be also modified by sensory stimuli. A similar 

 condition of increased tonus is observed in the frog. 



The apparatus for emotional expression is still intact though somewhat 

 modified, and an impression which would give rise to pain in the intact 

 animal may cause vocalisation in an animal in whom the brain above the 

 mesencephalon has been destroyed. 



