6 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



skeleton may be felt imbedded if. the muscles. Identify them as follows with 

 the aid of Holmes (Fig. 63, p. 230). The head contains a bony case, the skull, 

 to which the upper jaw is immovably fused, while the lower jaw is hinged to it 

 by a joint. A bony arch, the pectoral girdle, supports the fore limbs. Feel this 

 on both dorsal and ventral sides at the level of the fore limbs. On the dorsal 

 side, the girdle terminates in a flat thin bone with a cartilaginous border, called 

 the suprascapula. Ventrally, the bones of the girdle, covered by the pectoral 

 muscles, are seen to be articulated to a slender chain of bones and cartilages 

 which occupies the median ventral line and ends anteriorly and posteriorly with 

 conspicuous rounded cartilaginous expansions. This whole structure is called 

 collectively the sternum, or breastbone (see Holmes, Fig. 67, p. 239). The hind 

 limbs are similarly supported by the pelvic girdle. Feel for this on both sides; 

 ventrally it forms a hard crest between the bases of the hind legs; dorsally 

 two of its bones, the ilium bones, extend forward, producing the two conspicuous 

 lateral ridges on the lower half of the back. The median dorsal line is depressed, 

 forming a groove. Pass the point of an instrument along this groove and feel 

 the vertebral column lying underneath, and its lateral expansions, the transverse 

 processes. The last pair of these has enlarged bulbous ends, which are really 

 rudimentary ribs, to which the anterior ends of the ilium bones, mentioned above, 

 are firmly attached. It is this region which produces the external hump in the 

 frog. Posterior to this point, the median line is occupied by a long slender bone, 

 which has been produced by the fusion of a number of vertebrae; it is called 

 the urostyle. 



The lateral and ventral abdominal walls are supported by muscles only, 

 skeleton being absent. There are several layers of these muscles, the external 

 layer consisting of, laterally, the external oblique muscle; ventrally on each side 

 of the median line, the segmented rectus abdominis muscle extending from the 

 pelvic girdle to the sternum; and between these, and partially covering the 

 external oblique, a posterior slip from the pectoral muscles. A flat, thin muscle, 

 the mylohyoid, extends transversely across the ventral side .of the lower jaw. 

 This arrangement of the ventral musculature is very similar to that of all verte- 

 brates, including man. The median ventral line from the pelvic girdle to the 

 posterior expansion of the sternum is occupied by a white strip, the linea alba, 

 under which there runs, in the frog, a conspicuous blood vessel, the anterior 

 abdominal vein. For further details and a picture of the foregoing features, 

 see Holmes (Fig. 70, p. 249). 



Cut through the muscles on the ventral side to the left (frog's left) of the linea 

 alba from a point just in front of the pelvic girdle up to the mylohyoid muscle, 

 cutting through the pectoral girdle. A large cavity, the body cavity, coelome, or 

 pleuroperitoneal cavity, which contains the internal organs, or viscera, is thus 

 exposed. This space is lined by a smooth shining membrane, the pleuroperi- 



