440 THE ANTIQUITY OF MAN 
bility of the mandible belonging to the skull ; but a 
certain degree of doubt is engendered. 
We now come to deal with a very important feature 
of the Piltdown man. He has developed to the very 
highest degree a character which most of us who have 
tried to unravel the geological history of man never 
expected to find in an early or primitive human type. 
That feature is the articular eminence — a pulley-like 
elevation on the anterior part of the joint for the jaw on 
the base of the skull (fig. 163). The articular eminence 
is one of the most ingenious of all mechanisms to be 
found in the human body. It is simple and effective. 
The manner in which the eminence brings about the 
opening of the mouth in a modern man is shown in 
fig. 163. The stippled lines show the position of the 
condyle, of the coronoid process, to which the temporal 
muscle is attached, and of the angle of the jaw, when the 
mouth is closed and the food is being ground between 
the teeth towards the end of a chewing movement. In 
that phase, the condyle of the jaw has ascended within a 
socket — the glenoid cavity — situated on the under-surface 
of the temporal bone, just in front of the ear. If the 
reader will place a finger in front of the opening of the 
ear he will feel the condyle enter its cavity as the lower 
teeth close against the upper. The position of the 
various parts of the jaw, when the mouth is opened, is 
also shown in fig. 163. A rotatory movement of the 
ascending ramus occurs as the mouth is opened. The 
condyle then mounts the articular eminence (fig. 163). 
A strong muscle, the external pterygoid, drags the 
condyle forwards on the eminence, thus depressing the 
body of the jaw and opening the mouth. It is a general 
law of the animal body — one established a hundred and 
fifty years ago by John Hunter — that no muscle can act 
by itself ; its opponents — the muscles which produce an 
opposite movement — must act at the same time to a 
moderate and yielding degree. The external pterygoid 
has three strong opponents — muscles which close the 
lower jaw and teeth against the upper jaw and teeth, 
