THE CENTRAL NERVOUS SYSTEM 737 



the right by the sight of food. Here an inhibitory influence must 

 have descended the fibres of the abducens, the only nervous path 

 connected with the extrinsic muscles of the left eye, and the relaxa- 

 tion of the left external rectus must have kept accurate step with the 

 contraction of the right internal rectus. Hering has made an 

 exhaustive analysis of the co-ordinated movements concerned in 

 opening and closing the hand in monkeys. These movements can 

 be produced by stimulation of the cortex or the internal capsule, but 

 not by stimulation of the anterior spinal roots. When the hand is 

 opened the muscles that open it are excited, and those which close 

 it are inhibited from the cortex. (See also p. 734.) 



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. 



The centre of gravity of the head is a little in front of the vertical 

 plane passing through the occipital condyles. A slight degree of 

 contraction 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 articula- 

 tion ; its support by muscular action alone would be an intolerable 

 fatigue, and the ligamentum nuchse 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 between the 

 ensiform cartilage and the eighth or tenth dorsal vertebra. The 

 perpendicular dropped from it passes a little behind the horizontal 

 line joining the two acetabula ; but the body is prevented from falling 

 backward by the tension of the ileo-femoral ligament and the fascia 

 lata, and perhaps by slight contraction of some of the muscles on the 

 front of the thigh. The perpendicular let fall from the centre of 

 gravity of the whole of the body above the knee passes very slightly 

 behind the axis of rotation of that joint, so that but little muscular 

 action is required to keep the knee-joints rigid. The whole weight 

 of the body is finally transferred to the astragalus on each side, the 

 perpendicular from the centre of gravity of the whole, which is 

 situated near the sacral promontory, falling a little in front of these 

 bones. By means of the muscular sense, and the tactile sensations 

 set up by the pressure of the soles on the ground, alterations in the 

 position of the centre of gravity, and consequent deviations of the 



47 



