546 



COLLEGE ZOOLOGY 



the 4 corpora quadrigemina derived from the 

 optic lobes are covered by the cerebrum; 

 the cerebellum is large and divided into 3 

 parts; the medulla is comparatively short. 

 These differences in brains are correlated 

 with the character of the activities of the 

 different vertebrate types and with the de- 

 gree of development of their senses. 



Human brain 



The human brain (Fig. 393) differs es- 

 pecially from that of all other animals in the 



relative size of the cerebral hemispheres and 

 the quantity of gray matter they contain. 

 The average weight of the human brain is 

 about 1350 grams, whereas that of a gorilla, 

 having approximately the same body weight, 

 is only 430 grams, and that of a dog of equal 

 weight only 135 grams. The cerebrum, 

 which is the seat of intelligence and con- 

 scious sensations, of memory associations, 

 and of the control of voluntary movements, 

 comprises about 80 per cent of the total 

 weight. The surface of the cerebrum is 



Longitudinal fissure 



Olfactory bulb 

 Olfactory tract 1 



Optic II 



Optic chiasma 

 Optic tract 



Oculomotor III 

 Trochlear IV 



Trigeminal V 

 Abducens VI 

 Facial VII 

 Glosso- 

 pharyngeal IX 

 Vagus X 

 Hypoglossal XII 

 Accessory XI (spinal root) 



Cerebellum 



—Frontal lobe 

 of cerebrum 



Pituitary body 



(hypophysis) 



Infundibulum 



Mammillary body 



Pons 



Temporal lobe 



of cerebrum 



Auditory VIII 



Medulla 



Occipital lobe 

 of cerebrum 



Figure 393. Brain of man, ventral surface. The numbers I to XII indicate the cranial nerves. 

 (After Gerrish.) 



thrown up into ridges or convolutions, 

 which increase the area tremendously. The 

 gray matter is correspondingly increased. 

 This gray matter consists of the bodies of 

 about 10 billion nerve cells. 



Localization of function in the brain 



The cerebrum, cerebellum, and medulla, 

 each has its own particular functions. Im- 

 pulses are conducted between the spinal 

 cord and the brain through the medulla. In 

 the medulla are groups of cells which in- 

 fluence the heart beat, blood pressure, and 



the rate and volume of respiration. Here also 

 various reflex activities are controlled, in 

 eluding sneezing, coughing, vomiting, wink 

 ing, and the movements and secretions of 

 the digestive tract. 



The cerebellum helps to maintain posture, 

 equilibrium, and the tone of voluntary mus 

 cles; none of its activities comes into con- 

 sciousness. If it is injured, normal voluntary 

 movements are impossible; for example, the 

 hand may have difficulty in grasping an 

 object; there may be difficulty in walking 

 due to inability to control the muscles of 



