FUNCTIONS OF THE BRAIN 913 



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

 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 

 non-muscular structures, the bones and ligaments. But muscles also 

 play an essential part, for it is only peculiarly-gifted individuals, like 

 some of the fishermen of the North Sea, who can go to sleep on their 

 feet, and a dead body cannot be made to stand erect. The condition 

 of equilibrium is that the perpendicular dropped from the centre of 

 gravity to the ground should fall within the base of support that is, 

 within the area enclosed by the outer borders of the feet, and lines 

 joining the toes and heels respectively. The centre of gravity alters 

 its position with the position of the body, which tends to fall whenever 

 the perpendicular cuts the ground beyond the base of support. 



In the comfortable and natural erect position the centre of gravity 

 of the head is a little in front of the vertical plane passing through the 

 occipital cond)'-les, and as much as 4 centimetres in front of the vertical 

 plane passing through the ankle-joints. A certain degree of contrac- 

 tion of the muscles of the nape of the neck is required to balance it. 

 When these muscles are relaxed, as in sleep, the head must fall forward, 

 and this is the reason why Homer or any lesser individual nods. In 

 animals which go upon all-fours none of the weight of the head bears 

 directly upon the occipito-atloid articulation; its support by muscular 

 action alone would be an intolerable fatigue, and the ligamentum 

 nuchae is specially strengthened to hold it up. 



The vertebral column is kept erect by the ligaments and muscles 

 of the back. The centre of gravity of the trunk lies almost vertically 

 over the horizontal line joining the two acetabula, but the centre of 

 gravity of the whole body is about the level of the third sacral vertebra, 

 and a little more than 4 centimetres in front of the vertical plane 

 passing through the ankle-joints. Equilibrium it maintained by con- 



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