MAMMALIA 279 



over the body ; and in the cells it is used to give rise to energy 

 by oxidation, or it is stored, or converted by further enzymotic 

 action into living protoplasm. 



The whole alimentary canal is thus to be looked upon as 

 an apparatus for obtaining and digesting the food so that 

 it can be taken up by the blood and distributed all over the 

 complex body. It is a tube almost entirely made of endo- 

 derm, as has been seen from a consideration of the development, 

 and at particular points its walls are provided with glands 

 that is to say, outgrowths which provide at these points a 

 copious and sufficient supply of the digestive fluids. 



Respiratory Organs. The lungs are the respiratory organs, 

 and with them are associated the windpipe and its branches, 

 and the nose. 



The nose is formed as a pair of involutions of the ectoderm, 

 and each becomes connected by a groove with the stomodeum. 

 The stomodeum is indented laterally by the mandibular pro- 

 cesses below and the maxillary processes above. The margin 

 of the nasal cavity also becomes prominent in processes, 

 medial and lateral. At this period the conditions are very 

 similar to those illustrated by the skate, a median mass inter- 

 vening between the two nasal involutions and the mouth 

 being the frontonasal process. The maxillary processes grow 

 medially and fuse in doing so with the lateral nasal processes 

 and then with the medial nasal processes, and fusion is con- 

 tinued internally, with the result that two shelves, palatine 

 processes, grow out which meet and fuse, dividing the nose 

 from the mouth. In cases of incomplete fusion of the upper 

 lip during these changes hare-lip results, and it is usually 

 associated with an incomplete fusion of the palatine processes, 

 thus giving rise to cleft palate. 



The two nasal passages thus formed are resolved into two 

 tubes lying between the palate and the brain case. They 

 are lined by a mucous membrane which is ciliated, moist and 

 warm, and is greatly increased in area by the presence of the 

 nasal and maxillary turbinals. The air passes through this 

 labyrinth to reach the pharynx via the posterior nares and is 

 filtered from suspended particles. Crossing the pharynx, it 

 is passed into the windpipe and so to the lungs. 



