NERVOUS INTEGRATIONS IN MAN 253 



purely reflex acts initiated from receptive sources outside 

 the active muscles themselves, but during the execution 

 of even highly volitional acts. One set of the proprioceptive 

 reflexes arising intrinsically in muscle reinforces the muscular 

 contraction. A pull upon a muscle, whether passively given 

 or occasioned by the active contraction of the muscle itself 

 stimulates tension organs in the muscle and its tendon 

 and these receptors tend to excite reflex contraction of the 

 muscle and can reinforce contraction already present. 

 Active contraction itself stimulates certain other end-organs 

 within the muscle connected with the muscle fibers them- 

 selves, and these again exert a reflex influence on the driving 

 .nervous center. The intrinsic reflex (proprioceptive) influence 

 developed by the muscle itself on the nerve center imme- 

 diately driving it consists probably of opposed influences 

 of excitation and inhibition in various degrees of balance. 

 This is a fundamental factor in the normal behavior of our 

 muscles and its loss by disease may cause grave impair- 

 ment of posture and movement, and especially of skilled 

 acts. 



Among the acts which, using these partially self-regulated 

 muscles, the nervous system of man, like that of many 

 lowlier organized beings, . integrates essentially reflexly, 

 is that of maintenance of the erect position. How essentially 

 reflex this act is becomes evident from the competent way 

 in which in its two habitual forms of standing and stepping 

 it goes on without making any continuous demand upon 

 our mental attention. Conversation may seem to engross 

 the mind wholly while we stand or walk. Aristotle and his 

 peripatetics promenaded while discussing philosophy. In 

 the reflex basis of standing and stepping the nervous reactions 

 of man resemble fundamentally those of the animals, save 

 for the important detail that in man the vertebral column 

 is balanced vertically and the forelimbs are free of the 

 ground. The essence of the reaction seems to be that the 

 superincumbent weight of the body tends to put tension 

 upon and thus to stretch certain of the muscles, so exciting 

 them through their own reflex arcs to reflex contraction. 

 A widely distributed set of muscles so placed as to antagonize 

 gravity in the erect attitude of the body is found to be 



