THE PILTDOWN MANDIBLE 435 
mandible — the part to which the muscles of mastication 
are yoked — the opening for the nerve and vessels, which 
enter the jaw to nourish the teeth, is seen (fig. 160). Its 
anterior sharp margin carries a projecting spine. At its 
hinder lower margin runs off a narrow groove in which 
lie the vessels and nerve for the supply of the mylo- 
hyoid muscle. The groove is known, therefore, as the 
mylo-hyoid groove. Both vessels and nerve come from 
the main structures which enter the jaw at the dental 
foramen. The vascular groove is often separated from 
the one for the nerve. If the inner aspect of the 
chimpanzee's jaw is examined (fig. 161) the dental 
opening is readily seen ; it is large, but there is no short 
spine on its anterior border and the mylo-hyoid groove 
takes its departure some distance behind the opening 
for the dental nerve. Exactly the same characters are 
seen in the Piltdown jaw ; in this respect, also, that 
mandible is simian. When we seek to appreciate the 
significance of this character, we note at once that it is not 
the mylo-hyoid groove which has shifted away from the 
dental opening. In the human jaw the dental opening 
lies 20 mm. behind the last molar tooth or more ; in the 
Piltdown jaw and in the chimpanzee's the distance is 
considerably less — only 1 5 mm. It is easy to see how 
the difference has arisen. During all the years of youth, 
when the milk teeth are being replaced and space is being 
provided for the accommodation of the three permanent 
molars, the ascending ramus of the jaw is undergoing a 
process of continuous reconstruction. The nature of the 
change can be realised from the mandible of the infant 
gorilla shown in fig. 160. The two milk molars are in 
place. In the course of time, space for three large 
permanent molars — a space of 45 mm. (r8 inches) — has to 
be established behind the last milk molar for the three 
permanent molars. To secure such space, new bone is 
laid down along the posterior border of the ascending 
ramus ; the anterior border is at the same time removed. 
Every year the ascending ramus is moved backwards 
a space ; those masons of the animal skeleton — the osteo- 
