SKIN 



349 



them give a slimy liquid that possesses shghtly the acrid property 

 found in the secretion of the skin of toads and newts. By its 

 evaporation it cools the frog, which in air is always at a slightly 

 lower temperature than its surroundings. The pigment in 

 the cells is expanded and contracted in varying conditions of 

 light, temperature, etc., and thus the colour of the frog changes 

 (p. 368). Cold, dark, or wet surroundings cause expansion of 

 the pigment and darkening of the skin. Warmth, light, or dryness 

 cause contraction. From time to time, under the stimulus of 

 secretions of the thyroid and 

 pituitary glands, the horny 

 outer layer of the epidermis 

 is shed and eaten by the frog. 



sp. c. 



d. ao. 



I. V. c. 



per. 



v.s.s. 



g. msnt. 



coe. 



Fig. 264. — A 

 section 



diagram 



of a 



through the abdomen 



transverse 

 of a 



l.s.s. 



mso. 



^,i,i„i,i„ .. *,«//,m/. .,/„. . muse. 



GENERAL ARRANGEMENT 



OF INTERNAL ORGANS 



Immediately below the skin 

 is a series of large spaces, the 

 subcutaneous tymph sacs (Fig. 

 264). Between the lymph 

 sacs the skin is bound down 

 to the underlying flesh by 

 tough, white connective tissue, 

 but in consequence of the 

 presence of the sacs it is much 

 looser than that of most animals. Below the sacs there is a body- 

 wall of muscles enclosing the coelom. Between the dorsal wall 

 of this and the muscles of the back is a pair of large dorsal 

 lymph sacs in which lie the kidneys. 



male frog. 



coe., Coelom ; d.ao., dorsal aorta ; d.s., dorsal lymph 

 sac ; d.s.s., dorsal subcutaneous lymph sac ; g., 

 gut ; i.v.c, inferior vena cava ; k., kidney ; l.s.s.. 

 lateral subcutaneous lymph sac ; msnt., mesen- 

 tery ; mso., mesorchium ; muse, muscular body- 

 wall ; n., spinal nerves ; per., peritoneum : sk., 

 skin ; sp'.c, spinal cord ; t., testis w., vertebra; 

 v.s.s. , ventral subcutaneous lymph sac. 



SKELETON : GENERAL ARRANGEMENT 



The skeleton of the frog is composed chiefly of bone, but con- 

 tains also a good deal of gristle or cartilage. Although it is built 

 on the general vertebrate plan it is very atypical ; most of its 

 peculiarities may be connected with the unusual mode of pro- 

 gression by leaping. 



