24 ELEMENTARY ANATOMY. [LESS. 



The nature and mechanism of the different kinds of joint 

 have been described in the Seventh Lesson of " Elementary 

 Physiology," as well as the different kinds of movement which 

 the jointed bones are capable of performing. Joints may be 

 (a) immovable, () mixed, or (c] movable. 



(a) When bones are immovably joined by an interdigi- 

 tation of their irregularly shaped margins (like the bones of 

 the roof of the skull), they are said to be joined by suture; 

 but they may also be immovably united by a ridge, or tongue, 

 on one bone being received into the grooved surface of 

 another bone. 



(ff) When the motion allowed is exceedingly slight (as 

 between contiguous pieces of the backbone or vertebrae), 

 adjacent plain surfaces are connected together by the addi- 

 tion of fibrous substance of one kind or another. 



(c] When the motion allowed is greater, the adjacent surfaces 

 of the bones are coated with smooth cartilage, and motion is 

 facilitated by a fluid called synovial* Sometimes a third 

 and separated cartilage (termed inter-articular] is placed 

 between the cartilaginous surfaces of the jointed bones. 



The most movable joints are those in which the adjacent 

 bones are articulated on the principle either of a pivot (like 

 that between the two uppermost bones of the neck), or of a 



hinge (like that of the elbow), 

 or a ball and socket (like that 

 of the shoulder). 



If one convex articulating 

 surface be globular, it is termed 

 a head; if it be elongated it is 

 called a condyle. If either of 

 these is borne upon a narrow 

 portion of bone, this latter is 

 FIG. 37. Articulation of a large called a neck. If a pulley-like 

 spine, by shackle-joint, with a surface is formed by such a 



bony plate (placed below) of . . . ,- , J A -, 



the skin of a Siluroid fish. juxtaposition of two condyles 



as to leave a depression be- 

 tween them, such an articular surface is named a trochlea? 



The anatomy of animals (or xoofomy) 3 as distinguished 

 from the exclusive study of man's own anatomy (or an- 

 thropotomy*} shows us that bones may be united in ways 



1 From avv, with, and d>6v, an egg. The fluid is- contained in a fibrous bag 

 or sac. 



2 From Tpo X o<;, a pulley; -rp^a, to run. 



3 i-uiov, an animal, and T^fiKi*, to cut. 



4 uvtfpwTrot;, a man, and rtpveiv, to cut. 



