124 ELEMENTS OF MAMMALIAN ANATOMY. 



once. The posterior ones move forward to take the 

 place of the anterior as these become worn out. The 

 tusks of the walrus are its canines greatly developed. 

 None of the typical modern Ruminants have incisors 

 in the adult state, though they are present in the embryo. 

 Palaeontology has demonstrated that the ancestors of 

 the Ruminants had well-developed upper incisors in the 

 later Eocene times. The occurrence of the rudimentary 

 upper incisors in the embryo furnishes evidence of a full 

 dentition in the ancestors of the ruminants, since the 

 individual embryological history is somewhat of a 

 recapitulation of the ancestral history of the race. 



THE PHARYNX. 



The pharynx is that portion of the alimentary canal 

 between the mouth and the esophagus. The structure 

 is well shown by making a sagittal section through the 

 head and neck. This is best done with a saw, after the 

 specimen is frozen by placing it out-of-doors during one 

 or two days of cold winter weather. 



There are seven openings into the pharynx : the two 

 posterior nares (Fig. 18), opening anteriorly from the 

 roof; a Eustachian tube on each side; the esophagus; 

 and the larynx, leading into the trachea. The mucous 

 membrane lining the pharynx contains many simple 

 microscopic mucous glands. 



VISCERA. 



The body cavity must be opened before the rest of the 

 alimentary canal can be displayed. The entire ventral 

 wall of the thoracic and abdominal cavities should be 

 cut away with the bone forceps and scalpel. The body 



