ii6 ELEMENTARY BIOLOGY. [CHAP. 



each neural arch and its successor : between 

 the atlas and the second vertebra and be- 

 tween the eighth and ninth vertebrae ; these 

 are almost obliterated by the approach of the 

 respective arches. 



d. The neural canal. 



a. Closed by the centra of the vertebrae beneath, 

 and incompletely by the neural arches on the 

 sides and above : its backward continuation 

 as the canal in the front part of the ridge of 

 the urostyle. 



(3. The communication with it, of the inter- 

 vertebral foramina (4. b. y), and the dorsal 

 intervals between the neural arches (4. c. y) : 

 also of the openings in the urostyle (3. e). 



c. The skull. The prepared bony skull of the frog 

 is difficult to understand, for two reasons ; 

 firstly, on account of the dried-up condition of 

 the cartilages of which, in the fresh state, it is in 

 part composed ; and, secondly, on account of 

 the tendency of many of its constituent bones 

 to become ankylosed together in the adult : the 

 following points can however be made out with 

 tolerable ease. Drawings should be made of 

 each aspect of the skull. 



i . Examine the posterior end of the skull. 



a. The large aperture (foramen magnum} in the 

 middle line, leading into the cranial cavity. 



b. The convex surface (occipital condyle), on each 

 side of the foramen magnum, which articulates 



