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electrodes to a point close by on the surface changes the movement in ways quite inexplicable by changed physical conduction of the current; c) if the cortical 'centre' for a certain movement be cut under with a sharp knife but left in situ, although the electric conductivity is physically unaltered by the operation, the physiological conductivity is gone and currents of the same strength no longer produce the movement, which they did; d) the time-interval between the application of the electric stimulus to the cortex and the resultant movement is what it would be if the cortex acted physiologically and not merely physically in transmitting the irritation. It is namely a well-known fact that when a nerve-current has to pass through the spinal cord to excite a muscle by reflex action, the time is longer than if it passes directly down the motor nerve: the cells of the cord take a certain time to discharge. Similarly, when a stimulus is applied directly to the cortex the muscle contracts two or three hundredths of a second later than it does when the place on the cortex is cut away and the electrodes are applied to the white fibres below.''
(2) Cortical Ablations. When the cortical spot which is found to produce a movement of the fore-leg, in a dog, is excised (see spot 5 in Fig. 5), the leg in question becomes peculiarly affected. At first it seems paralyzed. Soon, however, it is used with the other legs, but badly. The animal does not bear his weight on it, allows it to rest on its dorsal surface, stands with it crossing the other leg, does not remove it if it hangs over the edge of a table, can no longer' give the paw' at word of command if able to do so before the operation, does not use it for scratching the ground, or holding a bone as formerly, lets it slip out when running on a smooth
* For a thorough discussion of the various objections, see Fcrrier's 'Functions of the Brain,' 2d ed., pp. 227-234. and Francois-Franck's 'Lecons gur les Fonctions Mot rices du Cerveau '(1887). Lecon 31. The most minutely accurate experiments on irritation of cortical points are those of Paneth, in Pfluger's Archiv, vol 37, p. 528.—Recently the skull has been fearlessly opened by surgeons, and operations upon the human brain performed, sometimes with the happiest results. In some of these operations the cortex has been electrically excited for the purpose of more exactly localizing the spot, and the movements first observed in dogs and monkeys have then been verified in men.
Surface or when shaking himself, etc., etc. Sensibility of all kinds seems diminished as well as motility, but of this I shall speak later on. Moreover the dog tends in voluntary movements to swerve towards the side of the brain-lesion instead of giving straight forward. All these symptoms gradually decrease, so that even with a very severe brain-lesion the dog may be outwardly indistinguishable from a well dog after eight or ten weeks. Still, a slight chloroformization will reproduce the disturbances, even then. There is a certain appearance of ataxic in-coordination in the movements —the dog lifts his fore-feet high and brings them down with more strength than usual, and yet the trouble is not ordi
Flo. 5.—Left Hemisphere of Doe's Brain, after Terrier. A. thf Assure of 8ylviua. B, the crucial sulcus. O, the olfactory bulb. I, II, m, IV, indicate the first, second, third, and fourth external convolutions respectively. (1), (4), and (5) are on the sigmoid gyrus.
nary lack of co-ordination. Neither is there paralysis. The strength of whatever movements are made is as great as ever—dogs with extensive destruction of the motor zone can jump as high and bite as hard as ever they did, but they seem less easily moved to do anything with the affected parts. Dr. Loeb, who has studied the motor disturbances of dogs more carefully than any one, conceives of them en masse as effects of an increased inertia in all the processes of innervation towards the side opposed to the lesion. All such movements require an unwonted effort for their execution; and when only the normally usual effort is made they fall behind in effectiveness.*
* J. Loeb: 'Beltrttge ztir Physiologie des Grosshirns; PflQger's Archly, nxix. 808. I simplify the author's statement.
Even when the entire motor zone of a dog is removed, there is no permanent paralysis of any part, but only this curious sort of relative inertia when the two sides of the body are compared; and this itself becomes hardly noticeable after a number of weeks have elapsed. Prof. Goltz has described a dog whose entire left hemisphere was destroyed, and who retained only a slight motor inertia on the right half of the body. In particular he could use his right
Fio. (.—Left Hemisphere of Monkey's Brain. Outer Surface.
paw for holding a bone whilst gnawing it, or for reaching after a piece of meat Had he been taught to give his paw before the operations, it would have been curious to see whether that faculty also came back. His tactile sensibility was permanently diminished on the right side.* In monkeys a genuine paralysis follows upon ablations of the cortex in the motor region. This paralysis affects parts of the body which vary with the brain-parts removed. The monkey's opposite arm or leg hangs flaccid, or at most takes a Hiuall part in associated movements. When the entire region is removed there is a genuine and permanent hemiplegia in which the arm is more affected than the leg; and this is followed months later by contracture of the muscles, as in man after inveterate hemiplegia.* According to Schaefer and Horsley, the trunk-muscles also become paralyzed after destruction of the marginal convolution on both sides (see Fig. 7). These differences between dogs and monkeys show the danger of drawing general conclusions from experiments done on any one sort of animal. I subjoin the figures given by the last-named authors of the motor regions in the monkey's brain, t
In man we are necessarily reduced to the observation post-mortem of cortical ablations produced by accident or disease (tumor, hemorrhage, softening, etc.). What results during life from such conditions is either localized spasm, or palsy of certain muscles of the opposite side. The cortical regions which invariably produce these results are homologous with those which we have just been studying in the dog, cat, a^e, etc. Figs. 8 and 9 show the result of
* 'Hemiplegia' means one-sided palsy.
t Philosophical Transactions, vol. 179, pp. 6. 10 (18S3). In a later paper (ibid. p. 205) Messrs. Becvor and Horsley go into the localization still more minutely, showing spots from which single muscles or single digits can be made to contract
169 cases carefully studied by Exner. The parts shaded are regions where lesions produced no motor disturbance.
Fig. 8.—Right Hemisphere of Human Brain. Lateral Surface.
Those left white were, on the contrary, never injured without motor disturbances of some sort Where the injury to
Fio ».—Right Hemisphere of Human Brain. Mesial Surface.
the cortical substance is profound in man, the paralysis is permanent and is succeeded by muscular rigidity in the paralyzed parts, just as it may be in the monkey.