242 ANATOMY. 



2. GYNGL.IMUS. When two parts are so connected that the one 

 is inserted within the other at its origin, and stands in intimate 

 connexion with it only at two opposite points. The part turns 

 upon these two points as upon its axis. This therefore admits of 

 but one kind of motion, viz. that of its approaching to or receding 

 from the other part. It is thus that the , coxae and trochanter, 

 femora and tibia are connected, and the mandible with the head. A 

 more detailed description will more clearly explain the peculiarity 

 of this articulation. Upon examining the upper extremity of the 

 tibia, which has been removed out of its socket, we shall observe 

 upon the exterior as well as interior a precise semicircular furrow, 

 behind it a concentrical but smaller ridge, and beyond this a cir- 

 cular fossulet. The inner surface of the femora displays on each 

 side a ridge accurately corresponding with the furrow, beyond this 

 a furrow corresponding with the preceding ridge, and in the centre 

 a minute elevation, from which a small but very firm band passes 

 into the central fossulet of the tibia. This band appears to pierce 

 transversely through the hole in the tibia, and passing through the 

 opposite side to be affixed to the corresponding central elevation of 

 the femora. Thus, therefore, a very firm connexion and a secure 

 joint is produced. The articulation of the mandible is very similar, 

 but which is distinguished from it by the upper side of the mandible 

 having a semicircular ridge, and upon its under side merely a spherical 

 ball joint. 



3. ROTATION (rotatio). Is that kind of articulation when a cylin- 

 drical, ovate, or conical part is sunk into a cavity adapted to its 

 convexity. Both the inserted body and the cavity are drilled at one 

 spot, and are united around the aperture by means of a membrane : 

 besides which there are balls at both poles of the axis of rotation 

 adapted to corresponding sockets of the other part ; whereby a rota- 

 tion of the encompassed part upon its axis is made possible within 

 the corresponding cavity. This mode of articulation is found in the 

 coxae of the Coleoptera, Hymenoptera, Hemiptera, or more or less 

 evident in the hip-joints of all insects. 



4. A FREE ARTICULATION (arthrodia). Is when a conical part 

 is inserted in a corresponding cavity, both being pierced at one spot, 

 and united by membranes around the circumference of the cavity. This 

 mode of union, which is the most common of all, admits of the freest 

 motion upon all sides ; and, indeed, what is still more, the exsertion 



