Metazoa Rana. 271 



however, in a matrix consisting chiefly of phosphate of 

 lime. 



The cavity of the tooth (pulp cavity) is occupied, as has 

 already been stated, by a papilla of submucous tissue. Over 

 the surface of the papilla there lies a number of branched 

 cells, from each of which proceeds an especially long branch 

 which enters a dentinal tubule. When we come to speak 

 of the minute structure of bone, the meaning and function 

 of these fibres will become apparent. 



The only other organ we need mention in the buccal 

 cavity is the tongue. It has already been stated that, 

 contrary to the usual rule, the frog's tongue is free behind 

 and attached to the jaw in front. The organ is composed 

 of bands of muscle separated and at the same time bound 

 together by connective tissue. The surface of the tongue 

 is covered by mucous membrane, which is, however, elevated 

 to form papillae. There are also special, papillae, which 

 have to do with the sense of taste. These will be referred 

 to later on when the senses come to be discussed. 



We must now endeavour to gain some general idea of 

 the nature of the alimentary canal and the modifications 

 met with on its course. The pharynx opens into a wide 

 dilatable tube, the oesophagus, opening in its turn into a 

 thick- walled stomach (fig. 153). The stomach together 

 with the lower part of the oesophagus is partly hidden by 

 the liver, a large, brown lobed organ, lying ventral to the 

 alimentary canal, and attached to its wall just anterior to 

 the proximal or cardiac end of the stomach. On leaving 

 the stomach the alimentary canal narrows, and forms a tube 

 of uniform bore, the small intestine, which lies coiled on 

 the right-hand side of the lower portion of the abdominal 

 cavity. The small intestine bends forward along the wall of 

 the stomach, and encloses in the fold a long whitish organ, 

 the pancreas. Lying between the lobes of the liver is a 

 small round sac. of greenish colour if full, the gall-bladder. 

 From the gall-bladder a delicate tube, the bile-duct, passes 



