THE AXIAL SKELETON. 



533 



the veins by two pairs of lymph-hearts. The skin consists 

 of a two-layered (ectodermic) epidermis, and an internal 

 (mesodermic) dermis. The transparent outer layer of the 

 epidermis is shed periodically, and swallowed by the frog. 

 The dermis differs markedly from that of a fish, for there 

 is no exoskeleton, though this was present in the extinct 

 Labyrinthodonts ; there are multicellular glands, whose 

 secretion keeps the skin moist and is 

 in part poisonous ; and there is a 

 stratum of unstriped muscle fibres. 

 Pigment cells occur in the dermis, and 

 some extend between the cells of the 

 epidermis. The colour changes a little 

 according to the state of these cells, 

 the protoplasm expanding and con- 

 tracting partly through the direct in- 

 fluence of light and moisture on the 

 skin, partly by a more complex reflex 

 action in which the eyes, the brain, 

 and the sympathetic nervous system 

 are all implicated. In the larval sala- 

 mander 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 distinct lateral lines, but 

 of this regularity the adult retains little 

 trace, though it has many nerve-end- 

 ings and " touch spots " in various 

 parts of its skin. 



The axial skeleton. — The vertebral 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 between the 

 condyles. It has no transverse processes, and its arch is in- 

 completely ossified. Each of the next six has an anteriorly 

 concave or proccelous centrum, a neural arch surrounding 

 the spinal cord, a transverse process from each side of the 

 base of the arch, an anterior and a posterior pair of articular 



Fig. 226. — Vertebral 

 column and pelvic 

 girdle of bull-frog. 



/./., Transverse processes 

 of sacral vertebra ; //. , 

 ilium ; [f. t urostyle ; Fe., 

 femur; lsch. t ischiac 

 region. 



