STRUCTURE OF FROG 



653 



trolled biochemically. The influence of light reaching the 

 brain through the eyes prompts the discharge into the 

 blood stream of a hormone from the pituitary body under- 

 neath the brain. A chemical messenger is thus carried by 

 the circulating blood to the pigment cells in the skin. In 

 the larval salamander the pigment cell seems to contract 

 and expand as a whole, but this is not usually the case. 

 There are cutaneous blood vessels, by 

 means of which the frog can, to a 

 certain extent, breathe by its skin. 

 The tadpole has sensory cells in dis- 

 tinct lateral lines, but of this regularity 

 the adult retains little trace, though 

 it has many nerve - endings and 

 " touch-spots " in various parts of 

 its skin. 



The axial skeleton. — The verte- 

 bral column consists of nine vertebrae, 

 and an unsegmented urostyle or 

 coccyx. 



The first vertebra bears two facets 

 for the two condyles of the skull, and 

 an odontoid process which lies be- 

 tween the condyles. It has no trans- 

 verse processes, and its arch is 

 incompletely ossified. Each of the 

 next six has an anteriorly concave 

 or procoelous centrum, a neural arch 

 surrounding the spinal cord, a trans- 

 verse process from each side of the 

 base of the arch, an anterior and a 

 posterior pair of articular processes, 

 and a short neural spine. The 

 eighth vertebra has a biconcave or amphicoelous centrum. 

 The ninth is convex in front, with two convex tubercles 

 behind, and bears large transverse processes with which 

 the hip-girdle articulates. The urostyle, formed by the 

 fusion of several vertebrae, has anteriorly a dorsal arch 

 enclosing a prolongation of the spinal cord ; but both arch 

 and nerve-cord soon disappear posteriorly. The noto- 

 chord, around which the vertebral column has developed, 



Fig. 374. — Vertebral 

 column and pelvic 

 girdle of bull-frog. 



t.p.. Transverse processes 

 of sacral vertebra ; //., 

 ilium; C/., urostyle ; Fe., 

 femur ; Isch., ischiac 

 region. 



