THE SKELETON. 4! 



with one another. At the extremities of the bones the 

 place of the medullary canal is taken up by cancellous 

 tissue (Fig. 13, d), the compact tissue being very thin. 

 The flat bones have no medullary canal, but the diploe 

 or cancellous tissue lying between the outer compact 

 tissue has its spaces rilled with a red marrow of the same 

 nature as that in the cancellous tissue of the long bones, 

 wherein the red blood-corpuscles are formed. The large 

 medullary cavity or canal is filled with yellow or fatty 

 marrow. 



BONES OF THE SKULL. 



The skull is usually considered in two parts, the 

 cranium and face. The former is composed of nine 

 bones, which will be described in order. 



The frontal bones (Figs. 15 and 16) are two in number, 

 lying between the orbits, and articulating with one 

 another in the median line. They form the roof of the 

 cephalic part of the brain cavity and the caudal portion 

 of the roof of the nasal chamber. A lateral projection 

 (po, Fig. 1 6) is known as the postorbital process. Within 

 the frontal bone is a cavity, the frontal sinus (Fig. 18), 

 which contains air and is lined with mucous membrane. 

 It communicates with the nasal cavity. The lateral 

 descending portion of the bone, articulating with the 

 palatine and orbitosphenoid, is the orbital plate of the 

 frontal. 



In the majority of the Mammalia the frontal is a 

 paired bone, but in man the two portions become anchy- 

 losed during the fifth or sixth year. The horns of 

 ruminants are outgrowths of these bones. Among the 

 Cervidae (deer) horns are usually developed only on the 

 male, and are shed every year. In the Bovidae (cattle) 

 the horns are permanent when present. 



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