352 THE FROG. PHYLUM CHORDATA 



The fourth has large transverse processes directed backwards 

 at an angle of about 45°. 



In the fifth, sixth, and seventh, the transverse processes are 

 directed slightly backwards. 



The transverse processes of the eighth resemble those of the 

 fifth to seventh but the centrum is amphicoelous, that is, it is 

 hollow both fore and aft. 



The ninth has large transverse processes directed backwards 

 to articulate with the ilia (p. 358) ; any vertebra which thus 

 comes in contact with the pelvic girdle is called sacral. The centrum 

 has one knob in front, for the eighth vertebra, and two behind 

 with which it articulates with the urostyle, which is a long, 

 tapering bone with a ridge above, and bearing in its front part 

 a canal for the hind part of the spinal cord. 



THE SKULL 



The skull of the tadpole consists entirely of cartilage, but in 

 the adult frog, as in the dog (p. 425), bones are formed in the 

 cartilage while other bones, called membrane bones, are applied 

 on the outside. Much cartilage, however, remains. The bones of 

 the skull are best learnt from behind forwards, with the sense 

 capsules considered after the brain-box. 



The cranium or brain-box is somewhat brick-shaped, and is 

 made mostly of cartilage (Fig. 267). At its hind end is a large 

 opening, the foramen magnum, through which the spinal cord is 

 continuous with the brain. On each side of this the cartilage is 

 replaced by an exoccipital bone, which has a backwardly directed 

 knob, the occipital condyle, but the upper and lower borders of 

 the foramen are not ossified. The only other bone in the cranium 

 is a large sphenethmoid at the anterior end ; it is shaped like a 

 dice-box, with across the waist a transverse partition which 

 makes the front wall of the cranial cavity ; the anterior hollow 

 is divided by a vertical partition. The sphenethmoid corresponds 

 to the mesethmoid, orbitosphenoids and presphenoid of the dog, 

 together with part of the nasal capsule. The roof of the car- 

 tilaginous cranium is incomplete, having three gaps or fontanelles, 

 one large anterior and two small posterior, but these are covered 

 by a pair of long membrane bones, the frontoparietals. Below 

 the floor, which is complete, is a dagger-shaped membrane bone, 

 the parasphenoid, with its handle directed backwards. 



