134 ELEMENTS OF MAMMALIAN ANATOMY 



Six molars are present on each side, only one or two of which 

 are functional at 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 upper incisors in the 

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

 tology has demonstrated that the ancestors of the Ruminants 

 had w^ell-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 some- 

 what 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 pos- 

 terior nares (Fig. 19), opening anteriorly from the roof; audi- 

 tory or Eustachian tube on each side; the esophagus; and the 

 larynx, leading into the trachea. The mucous membrane lin- 

 ing the pharynx contains many simple microscopic mucous 

 glands. 



VISCERA 



The body cavity known as the celom 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 cavity 

 is divided into two parts by the diaphragm (Fig. 61). The 

 cranial portion, the thoracic cavity, contains the esophagus, 

 heart, blood-vessels, thoracic duct, trachea, and lungs. In the 

 young cat there is present also the thymus gland, stretching 



