178 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



a substance of irritant nature. The latter, which are more 

 numerous and widely distributed, elaborate a slimy secretion 

 which makes the skin moist and slippery. These glands are 

 rounded vesicles, lying in the outer part of the dermis, but lined 

 by glandular epidermal cells. They are large and numerous. 



The dermis is mainly composed of fibrous* connective tissue, 

 the fibres of which are mostly parallel to the surface, but also 

 form vertical bands. The glands are imbedded in the external 

 part (1) of the dermis, which also contains a large number of 

 pigment-cells (chromatophores) that give to the skin its charac- 

 teristic colours. These vary owing to the fact that the chroma- 

 tophores contract under the influence of the nervous system. 

 In the fully contracted condition the pigment is limited to a 

 relatively small area, and the skin appears light. The reverse is 

 true when the chromatophores are non-contracted. Since these 

 colour-changes protect a frog by causing it to harmonize with its 

 surroundings, thus making it inconspicuous to its enemies, we 

 have here a case of protective general resemblance. But since also 

 the arrangement conceals the frog from its prey to a greater or 

 less extent, the general resemblance is aggressive as well as 

 protective. 



The glands are surrounded by unstriated muscle-fibres, which 

 also form a layer in the deeper part of the dermis, where also 

 occur networks of blood-vessels, lymphatics, and nerves. Many 

 of the nerve-fibres end in touch-corpuscles; small oval flattened 

 bodies mostly forming groups underneath the epidermal warts. 



3. The endoskeleton in the frog is mainly made up of gristle, 

 or cartilage, and bone, both of which are modifications of connective 

 tissue. The bones are largely connected at the joints by fibrous 

 bands (ligaments), which resemble tendons in structure (see p. 208). 

 Those parts of the endoskeleton belonging to the head and trunk 



* Connective Tissue This permeates the whole body, binding together the 

 other tissues. In higher animals generally it consists of three elements, 

 associated together in different proportions. These are (1) Connective-tissue 

 corpuscles, nucleated cells, often branched (of which pigment-cells are modi- 

 fications), most abundant in young tissues. (2) White fibres delicate, and 

 wavy, yielding gelatin on boiling. (3) Yellow elastic fibres, much branched, 

 or forming networks. Unaffected by boiling. 



The fibres are developed from the cells. All are imbedded in a structure- 

 less ground substance or matrix, which is semifluid and albuminous. 



All these elements are found in the skin and in the bands uniting it with 

 the body- wall. 



