440 THE RABBIT. PHYLUM CHORDATA 



arrangement is called diphyodont. The nomenclature of heteio- 

 dont teeth is based, as we have seen, on the bones which bear 

 them, but in most mammals each group has a characteristic 

 shape and function. The incisors are biting teeth, and in the rabbit 

 are chisel-shaped ; the canines are typically large stabbing 

 teeth, but the rabbit has none. The cheek teeth are used for 

 crushing and grinding, and have various patterns in different 

 groups of mammals ; in the rabbit they are all alike, with broad, 

 transversely ridged crowns. In some mammals the anterior 

 cheek teeth, which have milk predecessors and are called pre- 

 molars, are recognisably different from the posterior ones, called 

 molars, of which only one set occurs in the life of an individual. 

 The distinction of premolars and molars can only be made in 

 the rabbit by an investigation of their development. The numbers 

 of the teeth of a mammal are usually expressed by a formula, 

 which appears as a fraction, incisors, canines, premolars and molars 

 of one side of the upper jaw in the top line, and incisors, canines, 

 premolars and molars of one side of the lower jaw below. 



2033 



The dental formula of the rabbit is , so that there are 



1023' 



twenty-eight teeth in all. Between the incisors and the premolars 



is a long gap, the diastema. The dental formula for the dog is 



3142 

 , and the fourth upper premolar and first lower molar, 



which bite against each other, are specially enlarged as shearing 

 teeth or carnassials. The teeth of the rabbit grow throughout 

 life, but in most mammals the roots close and growth ceases when 

 the tooth is fully formed. 



Posteriorly the buccal cavity and the narial passage lead into 

 a short pharynx. Behind, this leads above (dorsally) into the gullet 

 and below to the glottis, which lies shortly behind the tongue, 

 covered by a flap, known as the epiglottis, which is stiffened 

 by a cartilage. Thus in the pharynx there cross one another the 

 passages by which the food passes to the alimentary canal and 

 the air to the lungs. When the animal swallows the soft palate 

 is raised and thus closes the posterior nares, while muscles contract 

 and close the opening of the windpipe, so that when the food is 

 thrust backwards by the muscles of the tongue and pharynx 

 it passes only into the oesophagus or gullet, a tube which runs 

 backwards through the neck and chest, above the trachea. The 

 epiglottis perhaps enables the animal to breathe while eating. 



