PSYCHIC FUNCTIONS OF THE CEREBRUM. 777 



the hemispheres cover the quadri geminate bodies entirely, but the cerebellum 

 still lies behind the cerebrum. Only in man do the occipital lobes of the cere- 

 brum entirely cover the cerebellum. 



According to Meynert these relations can be made clear in another manner. 

 It is well known that fibers pass downward from the cerebral hemispheres through 

 the cerebral peduncles, particularly their lower portion, which is known as the 

 crusta of the peduncle. This is separated by the substantia nigra from the upper 

 portion, which is designated the tegmentum and is connected with the quadri - 

 geminate bodies and the optic thalami. The larger, therefore, the cerebral hemi- 

 spheres the more numerous are the fibers passing to the crusta. In Fig. 254 at II 

 is shown a vertical section through the posterior quadri geminate bodies, including 

 the aqueduct of Sylvius, and the two cerebral peduncles from an adult man: 

 p p is the crusta of each peduncle, over which is the substantia nigra (s). Fig. IV 

 exhibits the same relations in the ape, Fig. Ill in the dog and finally Fig. V in 

 the guinea-pig. It will at once be seen that the size of the crusta diminishes in 

 the order named. In correspondence with this there is an analogous diminution 

 in the size of the cerebral hemispheres and at the same time of the intelligence 

 of the respective animals. 



Finally > the degree of intelligence is dependent upon the complexity of the 

 fissures in the hemispheres. While the fissures are yet wholly wanting in the 

 lower animals (fish, frog, bird, Fig. 258, IV, V, VI) two shallow fissures are present 

 on each side in the rabbit (III). The brain of the dog already exhibits numerous 

 convolutions (I, II). The complexity of the convolutions in the elephant, the 

 most intelligent of animals, is striking. Even in evertebrates, as, for example, a 

 number of insects endowed with delicate instincts, convolutions have been ob- 

 served in the cerebrum. Naturally, it cannot be denied that even some animals 

 of low intelligence, such as the cow, possess hemispheres with complex convolu- 

 tions. A similar condition has often been found in man in association with 

 marked mental development, although brains rich in convolutions have been ob- 

 served also in incompetent persons. In the male sex the average absolute weight 

 of the brain of the first two decades is greater than in females. The absolute 

 weight of the brain cannot be taken as an index of the degree of intelligence. 

 The elephant has the absolutely heaviest, man the relatively heaviest brain. 



The cerebrum consists in all vertebrates of three divisions: the olfactory lobe, 

 the striate body and the cortex. The olfactory apparatus is situated at the base 

 and is well developed in fish, although it varies greatly in size in verte- 

 brates. It is large in reptiles and small in birds. The striate body is pretty uni- 

 formly developed and serves as a means of connection between the optic thalamus 

 and the forebrain; from birds and mammals onward connections between the 

 thalamus and the cerebral cortex appear. The cerebral cortex is the most important 

 portion of the brain with respect to mental development. In the bony fish and 

 the ganoids it is represented by merely a thin epithelial plate. In reptiles a 

 cerebral cortex related to psychic activity first appears, but at the beginning there 

 is only an olfactory sphere. These animals are, therefore, the earliest that are 

 able to retain olfactory impressions in memory and to utilize them psychically. 

 In birds the visual sphere and the optic radiation appear first and these animals 

 are therefore the lowest that appreciate visual impressions psychically. In mam- 

 mals the other spheres are added. In mammals the relations between the sensory 

 and the sensorial nerve-paths to the cerebral cortex increase progressively. The 

 brain of the mammal, however, is characterized particularly by the remarkable 

 development of association-paths. 



Time-relations of Mental Processes. For the occurrence of mental processes 

 it is necessary for a certain time to elapse between the application of the stimulus 

 and the conscious reaction. This reaction-time, which is much longer than the 

 simple reflex time, can be measured by noting the moment of irritation, and then 

 having the individual under examination make a signal indicating the resulting 

 correct conception. The reaction-time will then consist: (i) Of the duration of 

 perception (entrance into consciousness) ; (2) of the duration of apperception 

 (consciousness of the special qualities of the sensation, such as form, pitch, color, 

 etc.), (3) of the duration of the voluntary impulse (for the making of the signal). 

 In addition there is to be taken into account (4) the time required in the propa- 

 gation through the centripetal nervous apparatus and (5) through the motor 

 nerve. If the signal is, as usual, given with the hand, the reaction-time for im- 

 pressions of sound is from 0.136 to 0.167 second, of light from 0.15 to 0.224 

 second, of taste from 0.15 to 0.23 second, of touch from 0.133 to 0.201 second. 



