180 



STUDIES IN PHYSIOLOGY 



cutting away the ligaments of this joint we see the smooth 

 surfaces of cartilage. Bet ween' them is a slimy liquid some- 

 thing like the raw white of egg. This is the syn-o'vi-al 

 fluid, which is secreted by the synovial membranes, that 

 cover the inner surfaces of the ligaments, and it serves to 

 lubricate the heads of the bones, and to prevent friction. 

 In its appearance and chemical composition, the synovial 

 liquid resembles more or less closely the blood serum, 



from which it is 

 prepared by the 

 synovial membrane. 

 Besides the liga- 

 ments which join 

 the bones, one sees 

 other cords of con- 

 nective tissue called 

 ten'dons. A tendon 

 is attached at one 

 end to a bone, and 

 at the other end it 

 becomes continuous 

 with masses of mus- 

 cle. By this means 

 the pull of the mus- 

 cle causes the move- 

 ment of the bone. 

 The tissues we have enumerated are present in the struc- 

 ture of each joint in the human body. A joint, then, must 

 have (1) at least two separate bones, (2) layers of cartilage, 

 (3) ligaments, (4) synovial membranes secreting synovial 

 fluid, (5) muscles and tendons. 



Classification of Joints. 1 The joints in the human body 

 may be divided into four classes, which are as follows: 

 (1) ball-and-socket, (2) hinge, (3) gliding, (4) pivot. 



A ball-and-socket joint, as the name implies, is formed be- 

 i See " Laboratory Exercises," No. 36. 



FIG. 77. Connective Tissue Fibers. 



a = small bundles of fibers. 

 b = larger bundles of fibers. 

 c = single elastic fibers. 



