374 MAMMALIAN ANATOMY 



VAKIATIONS IN FORM AND DEVELOPMENT. 



The uasal bone presents many variations in form, and these may 

 be regarded as modifications of several distinct types. 



We find flattened bones, in which the anterior surface is only 

 slightly curved above and the posterior median border is almost 

 straight ; strongly curved bones, in which the anterior surface is boldly 

 arched almost to the inferior border and the posterior margin is 

 deeply emarginate ; short, broad bones ; long, narrow bones, and 

 bones in which the diameters retain their normal relative propor- 

 tions, irrespective of their actual lengths. 



The superior border exhibits all varieties of outlines, except the 

 emarginate ; it may be transversely truncate ; obliquely truncate be- 

 hind ; gently and evenly arcuate ; obliquely rounded in front or 

 behind, or produced to a sharp point. 



The lower median angle of the external surface is, in some speci- 

 mens, so prominent that it forms with its fellow of the opposite bone 

 an obtuse superior nasal spine ; in other specimens it is truncated. In 

 the latter case the superior border of the nasal aperture is almost 

 straight. 



Not infrequently the external inferior angle of the bone is so broad 

 that it presents a short, sharp, inferior border, which is directed from 

 below obliquely upward and outward. 



The variations in the length of the ethmoidal fossa are shown in 

 the preceding table. 



HUMAN NASAL BONES. 



The external or facial surface of the nasal bone of man (Fig. 293) 

 is transversely more convex than the corresponding surface on the 

 nasal of the cat ; it is more deeply concave from above downward, and 

 faces laterally as well as forward and upward. It presents near the 

 middle a variable venous nasal foramen. 



The internal or nasal surface is deeply concave transversely, and 

 faces in great measure inward. It is traversed near the outer border 

 by a groove for the nasal branch of the fifth cranial nerve. 



The superior border is so broad that it might be termed a 

 roughened surface ; it articulates in the nasal notch of the frontal 

 bone. 



The inferior border is thin, and affords a line of attachment for 



