DIVISIONS OF THE DIGESTIVE SYSTEM 99 



mammalia; and in many cases it is further increased by the inde- 

 pendent elaboration of the blind intestine or caecum. In the rabbit 

 the combined length of the small and large intestines is approxi- 

 mately eleven times that of the body. 



Principal Divisions 



In considering the divisions of the digestive tube in the rabbit, 

 the posterior, or post-cephalic portion, comprising the oesophagus 

 and succeeding parts, may be distinguished from the anterior, or 

 cephalic portion, the latter comprising the oral cavity and pharynx. 

 The former is a free portion embracing the digestive tube proper, 

 while the latter is a fixed portion exhibiting a variety of general 

 mammalian features connected with the organization of the head. 



The form of the anterior, or cephalic portion of the digestive 

 tube (Plate II) depends on its fixed relation with respect to the 

 enclosing parts of the head-skeleton. In the rabbit, as in mammals 

 generally, the oral cavity is divisible into two portions, of which 

 one is the oral cavity proper, while the other, the vestibulum oris, 

 is a space enclosed between the alveolar processes of the jaws 

 and the teeth on the one hand, and the cheeks and lips on the other. 

 As in other vertebrates, the tongue is a muscular structure pro- 

 jecting upward and forward into the oral cavity from its base of 

 attachment on the hyoid apparatus, but its greater elaboration and 

 the differentiation of special processes, the circumvallate and foliate 

 papillae, for the accommodation of the gustatory organs, are fea- 

 tures characteristic of mammals. The rooif of the oral cavity is 

 formed by an extensive palatal surface, comprising the hard palate, 

 and the membranous, or soft palate. These structures also form 

 the floor of the accessory respiratory tracts of the nose, the posterior 

 aperture being thus carried backward to a point almost directly 

 above the aperture of the larynx. 



The chief features of the pharynx depend on the fact that it 

 is not merely a simple portion of the digestive tube but is 

 also related structurally and functionally with the tubes of the res- 

 piratory system. It is divisible into an oral portion, representing 

 the direct connection of the oral cavity with the oesophagus, a 

 dorsal or nasal portion, connected with the nasal fossae, and also 

 with the middle ear through the internal auditory tube, and a 



