THE FUNCTIONS OF THE BRAIN STEM 443 



guard and preside over the working of the mechanisms at the anterior 

 opening of the body ; by means of which food is seized, tasted, taken 

 into the alimentary canal, and finally digested. The respiratory 

 apparatus belongs to the same system and is innervated through 

 the same nerve channels. Hence the various events in alimentation, 

 such as deglutition, vomiting, mastication, or in the allied respiratory 

 functions, such as phonation, coughing, and respiration itself, are 

 endowed with centres in this part of the brain. In 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 animal 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 



