LOCOMOTOR MECHANISMS 173 



gradually pushed through the os into the vagina, followed 

 by the more easily passing remainder of the body. . After 

 expulsion of the fetus the contractions gradually diminish, 

 becoming painless, and in about fifteen minutes the after- 

 birth, consisting of placenta, amnion, chorion, decidua reflexa, 

 and parts of the decidua vera appears. At this time the blood 

 flows freely; the average loss, amounting to about 400 grams, 

 which can be and should be greatly reduced by a proper "follow- 

 ing down" i. e., intermittent massage of the fundus. After 

 parturition, by a process of involution lasting for several weeks, 

 the uterus returns to its unimpregnated state. The entire 

 process is a reflex act, the nervous centre being in the lumbar 

 portion of the cord. The nerves reach the uterus in company 

 with bloodvessels, and are derived from the pelvic plexus. 



Locomotor Mechanisms. The two hundred or more bones 

 of the body are joined together to form articulations of four 

 types: Sutures, symphyses, syndesmoses, and diarthroses. 



A suture is formed when two bones gradually interlock im- 

 movably, leaving only a more or less distinct seam. The best 

 examples are in the skull. 



A symphysis is the union of two bones by fibrocartilage, 

 as in the case of vertebrae. This allows a limited amount of 

 movement. 



A syndesmosis is a union of two bones by fibrous bands, 

 which allows considerable movement as in the inferior tibio- 

 fibular articulation. 



A diarthrosis is a union between two bones' that allows the 

 greatest movement, generally, however, only in a special 

 direction. The parts in contact are lined with cartilage, and 

 lubricated with a viscid synovial fluid. The union is made 

 firm by guiding ligaments and fibrous capsules. In some 

 cases, as in the head of the femur and acetabulum, the parts 

 fit so well that they are kept in place, partially, at least, by 

 atmospheric pressure. 



The movements between joints may be: (1) Angular; (2) 

 of circumduction; (3) of rotation; (4) gliding. In the first the 

 angle between the long axis of the bones changes in value. 

 In the second the longitudinal axis of the bone forms the sides 

 of ,M cone whose apex is at the joint. In the third the bone 



