THE FROG — A REPRESENTATIVE VERTEBRATE 397 



The lipophores reflect yellow light back through the epidermis. The rest 

 of the light penetrates to the guanophores, is dispersed, and blue light is 

 reflected back. The remaining light rays are absorbed by the melano- 

 phores. Yellow and blue light reflected back together result in a greenish 

 color. 



Changes in the general color tone of the skin are effected by the 

 migration of pigment within the melanophores. W'hen the skin darkens, 

 pigment streams out into the processes of these cells, some of which 

 mask the guanophores; when it becomes paler, the pigment concentrates 

 near the center of the melanophores. It is the pigment that migrates; 

 the processes of the melanophores remain extended. The movement 

 of the pigment is controlled in part by the hormone intermedin secreted 

 by the pituitary gland (page 622). 



182. Skeleton 



The skeleton of vertebrates forms the supporting framework of the 

 body, provides a point of attachment for most of the muscles, and encases 

 and protects much of the delicate nervous system. 



The somatic skeleton is the skeleton of the "outer tube" of the 

 body and is located in the body wall and appendages. It includes an 

 axial portion lying in the longitudinal axis of the body (vertebral col- 

 umn, sternum and most of the skull), and an appendicular portion 

 supporting the paired appendages (girdles and limbs). The visceral 

 skeleton is the skeleton of the "inner tube" of the body, and is associated 

 with the anterior part of the digestive tract. It is prominent in fish where 

 it supports the gills and helps to form and sujiport the jaws. In ter- 

 restrial vertebrates it is reduced, but parts of it remain associated with 

 the jaws, and parts become associated with the ear, tongue and larynx. 



Skull and Hyoid. The anterior end of the axial skeleton, together 

 with certain parts of the visceral skeleton, forms the skull, a complex of 

 bone and cartilage encasing the brain and major sense organs, and 

 forming the jaws. The central portion of the skull surrounding the brain 

 is known as the cranium; its more peripheral parts constitute the facial 

 skeleton (Figs. 21.3 and 21.4). The nasal cavities are situated near the 

 front of the skull; a pair of large openings for the eyes, orbits, lie lateral 

 to the middle of the cranium; and the inner part of the ears, containing 

 the receptive cells, lie in posterolateral extensions of the cranium known 

 as the otic capsules. A slender bony rod, the stapes, extends laterally 

 from each otic capsule. It is a part of the visceral skeleton which has 

 become modified to transmit vibrations from the tympanic membrane 

 to the inner ear. The spinal cord passes through a large hole, foramen 

 magnum, at the posterior end of the cranium. A pair of rounded bumps, 

 occipital condyles, lie ventrolateral to the foramen and articulate the 

 skull with the vertebral column. 



The upper jaw bears teeth along its margin and two patches of 

 vomerine teeth are borne by the vomer bones in the roof of the mouth, 

 but the lower jaw lacks teeth. The jaw joint lies between a quadrate 

 cartilage of the upper jaw and Meckel's cartilage of the lower jaw; 



