EFFECT OF GRAVITY ON POSTURES. 147 



as non-living things. The general effects of gravity 

 as affecting the growth and development of animals 

 have been referred to by many authors, but the effect 

 of gravity as a stimulus upon growing or mobile 

 parts in animals has been but little investigated. In 

 plants gravity influences growth in such a manner 

 as to produce various movements.* 



Gravity may be a factor in causing postures in 

 the human body in two different ways. 



Gravity produces a tendency in a limb to fall 

 downwards, i.e. to place itself with its centre of 

 gravity as low as possible. Thus, when the muscles 

 of a man's arms are not stimulated to action by 

 nerve-currents, and still the man stands erect, the 

 arms, under the influence of gravity, fall by his 

 sides. When in a strong man the head bends to 

 one side, it tends to become erect again ; that is, 

 the muscles tend to resume such a balance that 

 they are in equal tension on the two sides. This 

 may receive some explanation on the supposition 

 that when gravity causes the muscles on the convex 

 side to be strained, a nerve-stimulus is received by 

 those muscles through reflex action causing them 

 to contract. It must, however, be borne in mind 

 that the facts concerning growth of seedling plants 

 show that living cells may be directly stimulated 

 to certain kinds of action by gravity. 



The face is affected by the action of gravity 



when paralyzed or passive. The jaw drops when 



the masticatory muscles are relaxed, and when the 



facial muscles are paralyzed, the tissues of the face 



* Prantl and Vines. " Text-book of Botany," p. 87. 



