FUNCTION OF THE MUSCLES IN THE BODY. 583 



portion. Braune and Fischer found in the course of their investigations that 

 flexion at the knee-joint is attended with rotation of the tibia. The transition 

 from a position of extension to one of flexion of 20 is attended with an internal 

 rotation of 6. From this point further flexion is attended with an external 

 rotation, which amounts to 6 at a flexion of 90. 



4. Joints with Rotation about One Fixed Point. These are the freely movable 

 ball-and-socket joints (arthrodia). Movement is possible about innumerable axes, 

 all of which intersect at the point of rotation. The one articular surface has an 

 approximately spherical shape, while the other has that of a hollow sphere. The 

 shoulder- joint and the hip- joint are types of this articulation. Instead of the 

 numerous axes, about which movement is possible, three may be substituted, 

 intersecting at right angles in space. Therefore, these joints have also been 

 designated tri-axial. The movements possible are : (i) pendulum-like movements 

 in any desired plane; (2) rotation about the longitudinal axis of the extremity; 

 (3) movements circumscribing the surface of a cone, the apex of which corre- 

 sponds to the center of rotation of the joint, and whose surface is circumscribed 

 by the extremity itself. 



Limited artkrodial joints are ball-and-socket joints with a more limited range 

 of movement, and in which, moreover, rotation about the longitudinal axis is 

 wanting; for example, the metacarpo-phalangeal joints. 



5. Rigid joints (amphiarthrosis) are characterized by the fact that movement 

 is possible in all directions, but is limited in extent, owing to short and unyielding 

 external articular ligaments. The articular surfaces are usually of the same size, 

 and are almost flat. Examples are afforded by the articulations of the carpal and 

 tarsal bones with one another. 



With regard to the mechanical origin of the articular forms of two bones 

 movable upon each other, it is to be noted that the articular extremity to which 

 the muscles are inserted near the joint becomes the acetabulum; while that ex- 

 tremity to which the muscles are inserted at a greater distance becomes the head. 



Symphyses, synchondroses, and syndesmoses represent the junction of bones 

 without the formation of an articular cavity. They are movable in all directions, 

 but only to an extremely limited extent. Physiologically, they are thus closely 

 related to the amphiarthroses. 



Sutures unite bones without permitting any yielding. The physiological 

 significance of sutures resides in the fact that the bones may grow at their mar- 

 gins, so that distention of the cavity enclosed by the bones is possible. 



ARRANGEMENT AND FUNCTION OF THE MUSCLES IN THE 



BODY. 



The muscles form 45 per cent, of the total mass of the body. The 

 musculature on the right side of the body is heavier than that on the 

 left. If the muscles are considered with regard to their function from 

 the mechanical standpoint, the following categories may be distin- 

 guished : 



A. Muscles without Definite Origin and Insertion. 



T. The hollow muscles, enclosing spherical, oval, or irregular cavities, 

 such as the urinary bladder, the seminal -vesicle, the gall-bladder, the 

 uterus, the heart; or forming the walls of more or less cylindrical canals, 

 such as the intestinal tract, the muscular ducts of glands, the ureters, 

 the oviducts, the vasa deferentia, the blood-vessels, and the lymphatics. 

 Under such circumstances the muscle-fibers frequently are arranged in 

 several layers, for example in longitudinal, circular, and oblique direc- 

 tions. During activity all of the layers contract to effect diminution in 

 the size of the enclosed cavity. It is inadmissible to ascribe different 

 individual mechanical effects to the various layers, for example to main- 

 tain that the circular fibers of the intestine narrow the tube, while the 

 longitudinal fibers dilate it. Both sets of fibers rather act together in 

 diminishing the enclosed cavity, namely by narrowing and shortening it. 

 If, however, the wall of a hollow organ is pushed or folded inward either 



