THE NERVOUS SYSTEM 477 



which the brain communicates with the periphery and the peri- 

 phery with the brain. It gives origin to, or is connected with, 

 all the cranial nerves but those of smell, vision, and the muscles 

 of the eyeball. It is the supreme reflex centre for all the im- 

 portant functions of life, such as respiration (p. 133), circulation 

 (p. 84), the action of the heart (pp. 52, 57), and the digestive 

 apparatus from the mouth to the large intestine. It. is 

 astonishing that these varied functions can be controlled by a 

 few inches of nervous tissue. If the medulla be cut off from the 

 brain in front of its various centres, the animal continues to live, 

 respirations are regulated, the blood - pressure is maintained, 

 and the heart continues under control. If the section be made 

 behind the centres, all is at once changed, because the respirations 

 cease instantaneously ; the blood-pressure sinks dangerously, 

 but then partially recovers, rising in the cat and dog to a height 

 of about 90 mm. (instead of the normal 130). The heart beats 

 slowly but well, and the animal dies from asphyxia (see p. 134). 

 If artificial respiration be established, the beat of the heart 

 may be maintained for a day or more. The animal is conscious, 

 feels pain if the lip be pinched, blinks when the finger approaches 

 the eye, feels hunger, and will even seize food within its reach, 

 masticate and swallow it (Chauveau). 



The centres located in the bulb are those for mastication, 

 swallowing, sucking, vomiting, respiration, phonation, coughing, 

 the regulation of the heart -beat and arterial calibre, movements 

 of the iris, the secretion of saliva, the glycosuria centre, a centre 

 for the sweat glands, and a centre for shivering. It is not pro- 

 posed to deal here with these centres ; they have received 

 sufficient notice in the various chapters in which the above 

 questions have been considered. 



The Mid-Brain is composed of the corpora quadrigemina and 

 cerebral peduncles ; these form the basal ganglia. 



The Corpora Quadrigemina. — The white matter of the anterior 

 of these bodies is derived from the optic nerve, the fibres of 

 which arborise around its cells. Fibres from the corpora quadri- 

 gemina also run backwards to the cord, and in man gain a tract 

 known as the infero-lateral descending. The anterior pair of 

 bodies is concerned in vision. It forms a reflex arc connected 

 with the movements of the pupil, while, through its connection 

 with the optic nerve, it works upon the muscles of the eyeball. 

 The posterior corpora quadrigemina are important in the higher 

 group of vertebrates — viz., birds and mammals which have 

 a cochlea — that is, a ■ hearing ' ear — besides an ' equilibrating ' 

 ear. They receive fibres from the cochlear nerve, and have reflex 

 centres concerned with the lower auditory functions and with 

 vocalisation. 



