592 SITTING, WALKING, RUNNING, JUMPING. 



intersection of the three planes, passing in each instance at right angles to and 

 along the balanring edge. The center of gravity of individual parts of the body 

 may be determined in a similar manner on sections of a frozen cadaver. 



Pathological. The security of firm station is recognized from the swaying of 

 the body, which may be easily registered with the aid of a small rod placed verti- 

 cally on the top of the head, the swaying being recorded by means of a pen or 

 a brush on a surface stretched horizontally above the head. Disturbances of 

 sensation, such as occurs in tabes and the like, cause marked swaying; as do 

 also muscular weakness, tremor, fatigue, coldness of the feet, the action of an- 

 esthetics on the soles of the feet. 



SITTING. 



Sitting is the position of equilibrium in which the body is supported 

 on the tuberosities of the ischia, on which a to-and-fro rocking movement 

 can take place, as upon the rockers of a rocking-horse. The head and 

 the trunk together are made rigid so as to form an immovable column, as 

 in standing. The essential purpose of sitting is to place the lower ex- 

 tremities out of service from time to time, in order that their muscles may 

 recover from fatigue. The following varieties of the sitting posture 

 have been distinguished: i. The forward sitting posture, in which 

 the line of gravity passes in front of the tuberosities. In this position 

 the body is supported either against a firm object, for example by means 

 of the arms on a table, or on the upper surface of the thigh, which is 

 either held horizontally or is flexed to an acute angle at the hip by a 

 support placed under the feet. 2. The backward sitting posture is 

 characterized by the passage of the line of gravity behind the tuberosi- 

 ties. Falling backward is prevented under such circumstances by the 

 back of a chair (if the latter extends upward as far as the head the neck- 

 muscles also may undergo relaxation during rest), or by the counter- 

 weight of the legs, kept extended by muscular action. In the latter 

 event the sacrum may serve as a further point of support, while the 

 trunk is fixed on the thigh by the ilio-psoas and the rectus femoris, and 

 the leg is kept extended by the extensor quadriceps. Usually the center 

 of gravity is so situated that the heels form additional points of support. 

 This latter sitting posture is naturally not adapted for resting the mus- 

 cles of the lower extremities. 3. In the median sitting posture (sitting 

 erect) the line of gravity passes between the tuberosities. The muscles 

 of the lower extremities are relaxed ; the rigid trunk requires only slight 

 muscular action to balance it, falling backward being prevented by the 

 ilio-psoas and the rectus femoris, and falling forward by the lumbar 

 portion of the strong dorsal muscles. Usually, the balancing of the 

 head is sufficient to maintain equilibrium. 



WALKING, RUNNING, JUMPING. 



By walking is understood horizontal progression effected with the 

 least possible muscular exertion by alternate activity of the two legs. 



Method. The brothers William and Edward Weber, in 1836, analyzed the 

 various positions of the body during the movements of walking, running, and 

 jumping, and recorded these positions in continuous series, which thus represent 

 a true picture of all the successive phases of locomotion. Marey, in 1872, deter- 

 mined the time-relations attending change of position by connecting the motor 

 organs in man and animals with apparatus that registered by means of air-trans- 

 ference. He also further developed Weber's original idea, and has recorded the 

 various phases of movement in walking, running, and jumping, and in moving 

 animals by means of complete series of instantaneous photographs taken by a 

 camera working on the principle of the revolver. The duration of exposure in 



