FUNCTIONS OF THE BRAIN 945 



mammals after extirpation of one labyrinth. They are elicited with 

 special ease in the rabbit. In this animal it has been shown by study- 

 ing the movements by the aid of the cinematograph that the rolling 

 movements are really movements of progression (running and spring- 

 ing), which, on account of the changes in the position of the head and 

 neck, and in the tonus of the muscles, lead to a spiral rotation of the 

 body, in which the forward movement is small in proportion to the 

 rotation. 



In man, too, during the passage of a galvanic current through the 

 head by electrodes applied just behind the ears, a tendency to move 

 the head towards the anode is experienced. The person may resist 

 the tendency ; but if the current be strong enough his resistance will be 

 overcome ; he will execute a forced movement. When the head turns 

 towards the anode the eyes move in the same direction, and then under- 

 go jerking movements towards the kathode. There is at the same 

 time a feeling of vertigo. Complex as such an experiment is, involving 

 as it does stimulation of so many structures within the cranium, there 

 is reason to believe that it is the excitation of the semicircular canals, or 

 their cerebellar connections, that is responsible for these forced move- 

 ments. For when the experiment is performed on a pigeon, forced 

 movements are caused so long as the membranous canals are intact, 

 but not after they have been destroyed (Ewald). The observation of 

 Rawitz, that the peculiar rotatory movements of the so-called Japanese 

 dancing mice are associated with marked anatomieal peculiarities in 

 the labyrinth, is another fact in favour of the connection of the canals 

 with the maintenance of equilibrium and the sense of rotation. So is 

 the relation between the degree of development of the canals in different 

 species of birds and the degree of agility in the co-ordination of then 

 movements (Laudenbach) . 



But forced movements may also follow injuries (especially unilateral) 

 to many portions of the brain e.g., the pons, crus cerebri, posterior 

 corpora quadrigemina, corpus striatum, even the cerebral cortex, and 

 above all the cerebellum. The movements are of the most various 

 kinds. The animal may run round and round in a circle (circus move- 

 ment) ; or, with the tip of its tail as centre and the length of its body 

 as radius, it may describe a circle with its head, as the hand of a clock 

 does (clock-hand movement); or it may rush forward, turning end- 

 less somersaults as it goes. Intervals of rest alternate with paroxysms 

 of excitement, and the latter may be brought on by stimulation. In 

 man forced movements associated with vertigo have been sometimes 

 seen in cases of tumour of the cerebellum e.g., involuntary rotation 

 of the body in tumour of the middle peduncle. No entirely satisfac- 

 tory explanation of these forced movements has been given. They are 

 evidently connected with disturbance of the mechanism of co-ordina- 

 tion, leading to a loss of proportion in the amount oi the motor dis- 

 charge to muscles or groups of muscles accustomed to act together in 

 executing definite movements. For instance, in circus movements the 

 muscles of the outer side of the body contract more powerfully than 

 those of the inner side, and the animal is therefore constrained to trace 

 a circle instead of a straight line, the excess of contraction on the outer 

 side being analogous to the acceleration along the radius in the case 

 of a point moving in a circle. 



In connection with the consideration of the mechanism of equilibra- 

 tion, a short account of the muscular actions concerned in the main- 

 tenance of the erect posture so characteristic of man, and of those 

 concerned in locomotion, is subjoined here: 



Standing. In the upright posture the body is supported chiefly by 



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