MAMMALS. 573 



being never separate from its dental portion as a distinct bony 

 piece. The bone consists of two lateral parts, which meet in front 

 at the chin, but only in some mammals, as in man, entirely coalesce. 



Since the face includes the organs of sense, and by means of the 

 jaws is connected with the lower functions and necessities of life, 

 whilst the cranium on the contrary contains the brain, the noblest 

 part of the central nervous system, it becomes a matter of interest 

 to compare the relative size of these two divisions of the head : a 

 comparison which ought to give the same result as the investiga- 

 tion of the relation between the mass of the brain and that of the 

 cerebral nerves. To that end CAMPER devised the so-named 

 facial angle, which is determined by an oblique line (linea facialis) 

 drawn from the most prominent part of the forehead to the anterior 

 margin of the incisor teeth, or to the anterior extremity of the 

 intermaxillary bone, and by a horizontal line running from the 

 external auditory passage to the inferior margin of the nasal cavity. 

 Whilst the facial angle in man ranges from 70 to 85 degrees, it is 

 much more acute in the other mammals, and in monkeys reaches 

 only 40 degrees or more, whilst in other mammals it descends to 

 25 degrees, or even less. But since the facial angle can afford no 

 sure results unless the outer surface of the frontal bone be nearly 

 parallel to the inner, and this, from the great development of the 

 frontal sinuses, in many mammals is not the case, CuviER preferred 

 to compare the area of the cranial cavity with that of the facial bones, 

 as determined by sawing through the bony head longitudinally in 

 the mid plane. The ratio between these two areas, and so also the 

 facial angle, is modified by age; in the monkeys especially the 

 development of the posterior molar teeth presses the jaws forwards, 

 by which the head assumes a more animal character than in 

 younger individuals. The orang affords a striking example of this; 

 formerly the figures and the dimensions of the orang's skull were all 

 taken from young specimens ; from these were derived the values 

 of the facial angle given in elementary works ; but the facial angle 

 becomes much more acute, and all that is human in the character 

 almost entirely disappears in the Pongo of WURMB, which is now 

 known as the adult orang. 



Most mammals have teeth; the genera Manis and Myrmeco- 

 pliaga are however entirely destitute of them. In the duck-mole 

 (Ornithorhynchus) horny teeth are present; of the same kind are the 

 teeth of Eytina ILLIGER; they consist of tubules placed vertically, 



