INTERNAL STRUCTURE ^21 



each pleural cavity is known as a pleura, and of course covers 

 the lung as well as the inside of the thorax. The heart in its 

 pericardium does not lie free in the cavity of the chest, as that 

 of the frog does in the anterior part of the pleuroperitoneal 

 cavity, but is fastened to the dorsal and ventral walls of the 

 thorax by a double sheet of membrane, each sheet forming the 

 inner wall of a pleural cavity. Between the sheets is a lymph- 

 space known as the mediastinum. In the dorsal part of this space 

 lie the aorta, certain other blood vessels, and the oesophagus ; 

 its middle part is quite filled by the pericardium, with which its 

 walls fuse ; and in its ventral part lies the thymus. 



syn. c. 



ur.c.^ — 



SKELETON 



The skeleton of the rabbit has the general features which are 

 characteristic of vertebrates. It is predominantly an endo- 

 skeleton, situated internally, and is developed internally in 

 mesoderm (p. 187). For descrip- 

 tive purposes it is divided into 

 two parts, the axial skeleton 

 running the length of the body, 

 and the appendicular skeleton 

 attached to this. The axial 

 skeleton consists of the skull, 

 and the backbone or vertebral 

 column extending to the end 

 of the tail. In the thorax there 

 are, attached to the vertebral 

 column, ribs, which meet a 

 ventral axial piece, the breast- 

 bone or sternum. The separate 

 segments of the backbone are 

 vertebrae. The appendicular 

 skeleton consists of two hoops of 

 bone, the limb-girdles, to which the skeletons of the limbs are 

 attached. The structure of a joint or articulation, where one 

 part of the skeleton is movable on another part, is shown in 

 Fig. 334. The synovial capsule is a fibrous bag containing a 

 watery fluid, the synovia, the whole forming a spring and a 

 lubricant. In imperfect joints the synovial capsule is replaced by 

 a layer of cartilage or fibrous tissue, and the amount of movement 



Fig. 334. — A diagram to illustrate 

 the structure of ' perfect ' joints. 



ar.c, Articular cartilage ; bn., bone ; lig., liga- 

 ment ; med., medulla or marrow ; syn.c, 

 synovial capsule. 



