THE SKELETON. 23 



concussion and vibration. There is an exception, however, to 

 this usual connection in the temporal bones, which form the sides 

 of the cranial cavity, and which are connected to the other bones 

 by what is termed the squamous suture — one bone, in fact, 

 simply overlaps another. This union is inferior in strength to 

 the former; but nature has here another office to perform, and 

 the reason of this exception will at once be comprehended on 

 examininri; the skull. If a considerable blow be received on the 

 upper portion of the arch, its sides are the parts most likely to 

 give way ; and to guard against this consequence, the wider bone 

 overlaps the upper, and thus acts like the tie-beam of an arch in 

 keeping the parts together. This dovetailed suture does not 

 connect the bones of the inner table ; for though a carpenter 

 might find this mode of union serviceable in joining the sides of 

 a wooden box, a workman would by no means find it applicable 

 in connecting together brittle substances, as it would be ex- 

 tremely liable to chip off at the edges. 



The Cranial cavity, or that part which contains the brain, is 

 not more than a fourth the size of the other parts of the skull, 

 the remaining portions being devoted to mastication and smelling. 

 There are no less than nine bones which enter into the conqjo- 

 sition of the cranium. The two Frontal bones form the anterior 

 part, usually called the forehead ; but the internal plate of these 

 bones separates and recedes from the external plate so as to form 

 a cavity between them, which is called the frontal sinus, and is 

 divided by a septum or ridge of bone between them : the inter- 

 nal plate forms a covering for the anterior lobe of the cerebrum. 



The two Parietal bones are situated at the upper and middle 

 parts of the cranium, and cover the middle lobes of the cere- 

 brum, to which their internal part closely corresponds. These 

 bones become closely united after the second or third year. 



The Occipital, a single bone of great strength, is found at the 

 back and base of the cranium. Its internal surface covers the 

 cerebellum, and on a strong process at the base the medulla 

 oblongata rests. The external surface of this bone is extremely 

 irregular. At its upper part we find a crest or ridge of bone, 

 to which the cervical ligament is attached, as well as several 

 muscles of the neck. Below tliis is the occipital hole, through 

 which the spinal cord, as well as some nerves and an artery, 

 make their exit from the brain. On each side of this hole the 

 bone is smooth and rounded, for the purpose of articulating with 

 the atlas, the first bone of the neck ; besides which there are 

 several curious processes for the attachment of muscles. In the 

 foal this bone may be separated into four pieces. 



The Temporal bones, forming the sides of the cranium, are 

 composed of two parts, the squamous and the petrous. Though 

 in man these pieces are united, yet in the horse they are distinct 



c 4 



