SPECIAL PHYSIOLOGY OF MUSCLES. 57 



Sitting erect is a condition of equilibrium in which the body is balanced 

 on the tubera ischii, when the trunk and head together form a rigid column. 

 The vertical line passes between the tubera. 



Locomotion is the act of transferring the body, as a whole, through space, 

 and is accomplished by the combined action of its own muscles. The acts 

 involved consist of walking, running, jumping, etc. 



Walking is a complicated act, involving almost all the voluntary muscles of 

 the body, either for purposes of progression or for balancing the head and 

 trunk, and may be defined as a progression in a forward horizontal direction, 

 due to the alternate action of both legs. In walking, one leg becomes for the 

 time being, the active or supporting leg, carrying the trunk and head; the 

 other, the passive but progressive leg, to become in turn the active leg when 

 the foot touches the ground. Each leg, therefore, is alternately in an active 

 and a passive state. 



Running is distinguished from walking by the fact that, at a given moment, 

 both feet are off the ground and the body is raised in the air. 



While the limits of a compend do not permit of a description of the origin, 

 insertion, and mode of action of the individual muscles of the body, it has 

 been thought desirable to call attention to a few of the principle muscles 

 whose function it is to produce special forms of movement, as well as loco- 

 motion. (See Fig. 6.) The erect position is largely maintained by the fixa- 

 tion of the spinal column and the balancing of the head upon its upper ex- 

 tremity; the former is accompanied by the erector spines muscle, named from 

 its function and its fleshy continuations, situated on each side of the vertebral 

 column. Arising from the pelvis and lumbar vertebrae, this muscle passes 

 upward, and is attached by its continuations to all the vertebrae. Its action 

 is to extend the vertebral column and to maintain the erect position. The 

 head is balanced upon the top of the vertebral column by the combined action 

 of the trapezius and suboccipital muscles forming the nape of the neck, 

 and by the sterno-deido-mastoid muscle. This latter muscle arises from the 

 inner third of the clavicle and upper border of the sternum. It is inserted into 

 the temporal bone just behind the ear. Its action is to flex the head laterally 

 and to rotate the face to the opposite side. When both muscles act 

 simultaneously, the head and neck are flexed upon the thorax. 



The temporal and masseter muscles, situated at the side of the head, arise 

 respectively from the temporal fossa and the zygomatic arch, and are inserted 

 into the ramus of the lower jaw. Their action is to close the mouth and to 

 assist in mastication. The occipito-frontalis, the orbicularis palpebrarum, and 

 orbicularis oris muscles are largely concerned in wrinkling the forehead, closing 

 the eyes and mouth, and in giving various expressions to the face. 



