THORACIC APPENDAGES. 69 



The anterior coxae are the largest ; each consists of a flattened, almost 

 quadrilateral, tubular joint, loosely united by membrane to the pro-sternum. 

 A depression on its outer side lodges a strong tooth, borne by the edge of 

 the pro-sternum, upon which the whole limb moves, so that the joint 

 possesses a freedom of motion almost equal to that of a ball and 

 socket. Another smaller triangular plate is interposed between the 

 coxa and the anterior part of the sternum on the inner side of the limb. 

 This is probably the representative of the basal joint of the coxa, whilst 

 the larger part of the joint corresponds to the second article of the other 

 coxae. 



The femur is articulated with the coxa by a hinge allowing of move- 

 ment in only one direction ; all lateral motion being prevented by the edges 

 of the coxa overlapping the femur, and forming a groove, in which it moves. 

 A small accessory plate, closely united to the femur, protects the under side 

 of the joint. 



The intermediate coxae each consist of a basal segment, which is some- 

 what boat-shaped, but open both above and below. It is flattened 

 anteriorly, and pointed externally. Its anterior edge fits against the 

 posterior edge of the meso-sternum. A stout tooth, but not so large as 

 that which supports the anterior coxa, fits against its inner anterior 

 angle. It projects from the posterior edge of the meso-sternum, near the 

 mesial line. The joint between this part of the coxa and the meso- 

 sternum is a hinge joint, moving on its posterior edge, and giving the 

 limb a motion in an antero-posterior direction. 



The second joint of the intermediate coxa is short, and almost conical. 

 It is loosely connected with the last, and has a hinge-like motion upon its 

 posterior and inner edge, which gives the limb a considerable lateral play. 



The posterior coxae differ from the intermediate, in the first joint being 

 more nearly round, and in the absence of any well-marked tooth at the 

 inner and posterior part of the nieta-sternum. A very small tooth exists, 

 however, externally to the limb at the edge of the meta-sternum. These 

 teeth, both in the intermediate and posterior limbs, seem to act rather as 

 checks to the movement of the limb, preventing the thoracic muscles 

 from drawing the coxa into the interior of the thorax, than as pivots, upon 

 which the coxae move, like those of the anterior limbs. 



The joints between the coxae and femora in the posterior and intermediate 

 limbs, resemble the same in the anterior legs. The articulations between 

 the femora and tibiae allow of no lateral motion, the femur being grooved 

 vertically for the reception of the tibia. The tarsal joints allow of slight 

 lateral motion, the distal extremity of each being hollowed beneath into 

 a cone-like cavity, which receives the narrow proximal end of the next. 

 A similar joint exists between the proximal tarsal articulus and the tibia, 



