THE DIGESTIVE AND RESPIRATORY SYSTEM 271 



mouth, or hard palate. In fishes and amphibians this is formed 

 in great part by the anterior end of a single extensive median 

 bone, the parabasal, but in higher forms this element is reduced 

 and its function is assumed by the palatines and pterygoids 

 and by horizontal plates directed inward from the maxillaries 

 and premaxillaries. These latter elements are all lateral and 

 arise in the embryo upon the sides of the head and grow to- 

 wards the center, finally uniting in the median line. In some 

 groups of birds the two halves remain disassociated, thus 

 forming a palatine cleft by which a direct communication is 

 established between the nasal and pharyngeal cavities. This 

 failure to unite may occur as an abnormality in mammals, pro- 

 ducing the malformations known as hare-lip and cleft palate, 

 malformations thus attributable to the principle of arrested 

 development. In certain mammals, as the cat and dog, the 

 remains of the closure are permanently shown in the form of 

 a deep median groove, the philthrum, which partly divides the 

 upper lip and runs along the septum of the nose externally. 



In mammals the bony palate (hard palate), which is con- 

 posed of horizontal processes from premaxillaries and maxil- 

 laries and a portion of the palatines, is continued posteriorly 

 into a membranous soft palate or velum palati. This is a dupli- 

 cation of the pharyngeal mucous membrane and is supplied 

 with semi-voluntary muscular fibers from the sheets surround- 

 ing the pharynx, which are themselves probably derivatives 

 of the musculosa of the anterior portion of the alimentary 

 canaL 



Accompanying the stomato-pharyngeal division of the ali- 

 mentary canal are several important auxiliary organs derived 

 from various sources. These are the teeth, tongue, tonsils, the 

 glands of the mouth cavity, and the glands of the pharyngeal 

 pockets, which will be considered in the order given. 



Excepting the horny formation in the mouth of the Cyclo- 

 stomata, which are isolated structures of epidermic origin, 

 the teeth of the vertebrates are strictly homologous organs 

 throughout. They were originally placoid scales which dif- 

 ered in no respect from those that cover the exterior of the 



