102 THE STRUCTURE OF THE HORSE 



(embedded in the bones which form the side wall of the 

 brain-case), will be seen, to the roughened margin 

 of which the base of the cartilaginous ' pinna/ or pro- 

 jecting external ear, is attached. Although we com- 

 monly speak of this latter as the ' ear/ as it is the only 

 externally visible part of the complicated organ by 

 which sounds are recognised, it is a mere accessory, the 

 use of which is to aid in collecting the vibrations pass- 

 ing through the air, and direct them towards the in- 

 ternal, delicate, and beautifully-constructed apparatus 

 in which, by their effects upon the terminations of the 

 auditory nerve, they produce the sensation of hearing. 



In front of the ear-opening arises a curious bridge 

 of bone (2), which arches forwards to join the skull 

 again at the hinder part of the face. This is called 

 the 'zygomatic arch/ and is almost constantly present 

 in the skulls of mammals. 



Standing out as it does, it allows the upper part of 

 the under jaw to work beneath it, and its outer surface 

 affords a very advantageous point of attachment to a 

 great muscle (masseter), to be spoken of presently. The 

 side of the brain-case between this and the top ridge of 

 the skull is called the 'temporal fossa' (t). Though 

 bounded by raised ridges all round, enough to give it 

 the general character of a depression or fossa, its floor 

 is formed by a convex surface, the side of the actual 

 brain-cavity. The fossa is mainly filled in life by one 



