viii Mrs( I.K MOVKMI \ ra 77 



muscles to act in order to balance the action of the extensor 

 muscles. 



The trunk is supported on the thigh bones at the hip 

 joints by the balancing of the muscles passing from the 

 trunk to the thighs in front and behind. The trunk is some- 

 what heavier behind the hip joints than in front of them, and 

 so the tendency is to fall backwards, but this is prevented 

 chiefly by a strong ligament passing from the pelvis to the 

 thigh bone, over the front of the hip joint. The head is sup- 

 ported on the trunk and prevented from falling forward partly 

 by ligaments passing from the back of the head to the spinal 

 column. These ligaments are very strong in quadrupeds, as 

 in the horse for example. But it is by the contraction of the 

 muscles at the back of the neck that the head is supported in 

 exactly the right position. Directly a man, in a sitting pos- 

 ture, falls asleep, the head falls forwards. This is because the 

 contraction of the muscles at the back of the neck is no longer 

 maintained. How was the contraction maintained ? By 

 the brain, but this during sleep is at rest, and no longer 

 acting on the muscles. Again, if a man receives a violent 

 blow on the head, he will fall " in a heap," with his limbs 

 flaccid and powerless. All the muscles which were keeping 

 him upright have suddenly ceased their contractions. The 

 muscles are not injured, and after a time, the man will 

 " return to himself," regain consciousness, and be able to 

 stand upright as before. The blow on the head produced a 

 " shock " to the brain and spinal cord, and they ceased for a 

 time to keep the muscles contracted. The muscles, then, do 

 not contract of " their own accord," but only by the influence 

 of the central nervous system. The central nervous system is 

 in connection with the muscles by means of nerves, and the 

 muscles are controlled by means of impulses sent along the 

 nerves from the brain or spinal cord. 



