THE BONES OF THE HEAD. 299 



the bone ; for although externally we find the structure of 

 horn, internally we have bone, and between this horn and 

 bone we find the vascular structure by which both are secre- 

 ted. In many animals the age can be judged of by the horn, 

 which each year presents an additional ring round its base. 

 In the cow this is not an uncertain guide, and is owing to 

 the irregular growth of the horn at different periods of the 

 year, growing probably with greater force in the spring than 

 in the winter ; but in the sheep, although the same causes 

 obtain, yet it cannot be depended on with accuracy. At the 

 root of the horn we observe a cavity which communicates 

 with the frontal sinus. 



The two parietal bones are proportionately shorter than in 

 the horse, and are situated at the upper and middle parts of 

 the cranium, and cover the middle lobes of the cerebrum, to 

 which their internal part closely corresponds. 



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 lower and back part 

 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 pro- 

 cesses for the attachment of muscles. 



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 sheep 

 they are distinct from each other. The squamous portion is 

 externally a convex plate with a hooked projection arising 

 from it ; this process assists in forming the zygomatic arch. 

 The squamous portion affords at the posterior part a shallow 

 cavity for the articulation of the lower jaw-bone. This gle- 

 noid cavity, as it is termed, is much deeper in carnivorous 

 animals, which require to open their jaws more extensively : 

 and an inspection of this portion of the skeleton alone will 

 enable the comparative anatomist to decide to what order 

 the animal might have belonged. In herbivorous races a 

 grinding lateral motion of the jaws only is required, and ac- 

 cordingly the articulation is wide and shallow. The zygo- 

 matic arch, too, is much more arched in the carnivora, in order 



