THE ENCEPHALON OF VERTEBRATA. 



321 



with the cord, in the mammal, bird, amphibian, and fish. In the last two 

 cases it will be noticed that the brain weighs even less than the cord ; and it 

 is in such animals that we find the movements become more and more sensori- 

 motor or instinctive, or even purely excito-motor or reflex movements, which, 

 as we have already seen, are governed by the spinal cord, the medulla oblongata, 

 and their extensions upwards to the base of the cerebrum, rather than by the 

 cerebrum properly so called. 



Not only does the size of the entire encephalon become relatively less as 

 we pass from the highest to the lowest Vertebrata, but this part of the nervous 

 system undergoes a gradual simplification in its form and structure, more espe- 

 cially as regards the parts which, in this series of animals, represent the cere- 

 bral hemispheres and the cerebellum. These organs indeed, so decline in size 

 and complexity that they become gradually smaller in proportion to the sen- 

 sori-motor ganglia at the base of the cerebrum ; or in other words, as we de- 

 scend in the vertebrate scale these ganglia exhibit a greater proportionate size 

 as compared with the diminished cerebral hemispheres and cerebellum. With 

 the diminution of the cerebellum, there appear to be associated a diminished 

 complexity and variety of the muscular movements executed by the lower 

 Yertebrata ; whilst the remarkable defect in the cerebral hemispheres is accom- 

 panied by defective intelligence. 



The Cerebral Hemispheres. In the most intelligent mammiferous animals, 

 the anthropoid apes, these parts completely cover the olfactory nerves or lobes 

 in front, and the corpora quadrigemina behind, being indeed, as in man, pro- 

 longed so far backwards that they completely cover, and even overlap the 

 cerebellum. In many species of the still lower baboons and monkeys (Fig. 63, (/), 

 the amount of overlapping is even greater than in man. But in descending 

 through Carnivora, e, Cheiroptera, Ruminantia, and Pachydermata, and the 

 still lower Rodentia, the cerebral hemispheres no longer overlap, but soon 

 cease even to cover any part of the cerebellum, which ultimately is completely 

 visible when the encephalon is viewed from above. In the Ruminantia, /, the 

 anterior part of the hemispheres is also so proportionally diminished as to per- 

 mit the large olfactory lobes to project beyond them ; whilst in the Rodentia 



Fig. 63. 



Fig. 63. Brains of three of the Mnmmalia, to show the gradually increasing size and complexity of the 

 cerebral hemispheres, in the ascending scale of those animals, e, brain of the cat, showing the cerebrum, 

 and its few simple, almost exactly symmetrical convolutions: behind it is the much-lobulated cerebellum 

 and the medulla oblonjrata; and in front a portion of the olfactory lobes. /, brain of the sheep; the ollac- 

 tory lobes are almost hidden, and the cerebellum is about half covered by the cerebral hemispheres, which 

 are now more complex and less symmetrical, a, brain of a monkey, in which the olfactory lobes in front, 

 and the cerebellum behind, are completely overlapped by the cerebrum ; the convolutions ar now con- 

 structed on the plan observable even in the human cerebrum : a distinct posterior lobe can be recognized, 

 but the cerebrum is more pointed in front, its convolutions are more simple and symmetrical, and its rel- 

 ative size very much smaller than in man. 



(Fig. 64, cZ), owing to the still further diminution in the hemispheres, even a 

 portion of the corpora quadrigemina behind, becomes visible. In Birds, c, the 

 cerebral hemispheres overlap the small olfactory lobes in front ; but behind, a 

 very large portion of the optic lobes is visible. In the Reptiles and Amphibia, 

 6, a still further reduction of the cerebral hemispheres takes place. Lastly, in 

 the Fishes, a, they are relatively so small as merely to invest the corpora striata 



