In higher animals, such as rabbits and guinea-pigs, the cerebral hemispheres are comparatively much more developed than in the frog, and their removal interferes very much with the animal's motions. At first it is utterly prostrate, but after some time its power of movement returns to some extent, though it remains much less than in the normal animal. As we should expect, the weakness is most marked in those parts of the body that are most under the control of the cerebrum, and least in those whose movements are regulated by the lower centres. Thus in rabbits the fore-paws are capable of being used for complex motions at the will of the animal, such as washing the face, holding food, and so on, and in them the weakness caused by removal of the cerebrum is much more marked than in the hind limbs, which are simply used for progression. After the operation the animal can still stand, although it is unsteady, and the fore-legs tend to sprawl out. When pinched it bounds forward, but, unlike the frog, it is unable to avoid any obstacle in its path.

Fig. 67.   Diagram of the higher nerve centres of the frog.

Fig. 67. - Diagram of the higher nerve-centres of the frog.

opposite to the part of the brain irritated.

Fig. 68.   Brain of monkey, showing the position of the motor and sensory centres as ascertained by Ferrier. The actions all occur on the side of the body

Fig. 68. - Brain of monkey, showing the position of the motor and sensory centres as ascertained by Ferrier. The actions all occur on the side of the body

If it be pinched at all severely, it not only moves, but will cry loudly and plaintively, and this condition is frequently noticed in rabbits under chloroform, although they have received no injury whatever. The pupils contract on the stimulus of light, and the eyes wink if the finger is brought near them. Bitter substances cause movements of the tongue and mouth, and ammonia applied to the nostrils may cause the head to be drawn back, or the animal to rub its nostrils with its toes.1

Where the cerebral hemispheres are still more developed, as in cats, dogs, and monkeys, their removal causes so much prostration, and interferes so greatly with motor power as almost entirely to destroy equilibrium and co-ordinated progression.

The motor and sensory centres of the brain have been more exactly localised in monkeys by Ferrier, Fritsch, Hitzig, and others, and the results of their experiments, especially those of Ferrier, agree so well with those of pathological observation in men that we may assume that there is a general agreement between the position of the centres in man and monkey.

The motor centres are arranged along the two sides of the fissure of Rolando, the order of their arrangement being exactly what is required for the purpose of (1) seeing food; (2) conveying it to the mouth; (3) masticating it; (4) throwing away the refuse; and (5) advancing to get more2 (vide Fig. 68, brain of monkey).

The sensory centres lie in the posterior and lower parts of the brain. The centre for sight is situated in the angular gyrus and is marked 14 and 15 in the diagram; that for hearing is situated in the superior temporo-sphenoidal and is marked 16 in the diagram; those for smell and taste lie at the tip of the temporo-sphenoidal lobe, and the centre for general sensation appears to be towards the interior of the brain, in the hippo-campal region.

When the motor centres in the monkey are slightly irritated by a faradaic current, a single co-ordinated movement is produced, but if the irritation be continued longer, and especially if a strong current be used, epileptiform convulsions may occur, succeeded by choreic movements after the current has ceased. Epileptic convulsions are easily produced by irritation of the cerebral cortex in the cat and dog as well as the monkey. It is difficult to produce them by cortical irritation in the guinea-pig or rabbit, and impossible in birds, frogs, and fishes.3

1 Ferrier, Functions of the Brain, p. 38.

2 Lauder Brunton ' On the Position of the Motor Centres in the Brain in regard to the Nutritive and Social Functions,' Brain, vol. iv. p. 1.

3 Francois-Franck and Pitres, Arch. de Physiol., July 1883, p. 39.