CHAP. V.] BONE. 105 



os innominatum, all of which will be found to possess the same 

 essential characters. 



The cranial bones present one or two peculiarities which demand 

 a special notice. The layers of compact tissue in them are fami- 

 liarly known as the tables of the skull : the outer one is stouter and 

 tougher; the inner one denser and much thinner, and therefore 

 more brittle. The intervening structure is called the diploe ; in 

 some places it is absent, leaving a vacant space produced by the 

 separation of the tables, and which communicates with the external 

 air, as in the frontal sinuses; the diploe is generally a very fine 

 cancellated texture ; but, in the mastoid process of the temporal 

 bone, it is of a much looser kind, its cancelli are larger, and instead 

 of being occupied by medulla, as elsewhere, they communicate 

 with the cavity of the tympanum, and are therefore always filled 

 with air. The diploe of the cranial bones in birds is everywhere 

 devoid of medulla, and occupied by air, which gains access to it 

 from the tympanum. 



A fourth group of bones consists of those, which seem to combine 

 many of the offices and forms of the three preceding ones with 

 certain characters proper to themselves. They exhibit much irre- 

 gularity of shape and size; and, on this account, are called irregular 

 bones. The vertebrse, the tarsal and carpal bones, certain bones of 

 the head and face, belong to this group. Lightness, with extent of 

 surface, are their principal characters. They are composed mainly 

 of cancellated texture, covered by a layer of compact, and here and 

 there a portion of compact tissue, for the purpose of affording a firm 

 bond of connexion of some process to the main part of the bone; as 

 the pedicles, uniting the laminae to the bodies of the vertebrse. 



In examining the surfaces of these different groups of bones, we 

 are struck with the variety of projections or eminences, and of 

 depressions, which are found upon them. These are of two kinds : 

 articular, and non-articular. The former are destined for the 

 formation of joints : as the head of the thigh-bone, an articular 

 eminence ; and the acetabulum, an articular depression. 



The non-articular eminences chiefly serve as points of insertion 

 for ligaments and tendons, and exhibit a great diversity of shapes, 

 so that anatomists designate them as tuberosities, tubercles, spines, 

 cristae, &c. The non-articular depressions serve a similar purpose, 

 and are equally various in form, being described as fossae, cells, 

 furrows, grooves, fissures, pulleys, &c. 



With reference to these eminences and depressions, it may be 

 observed that they are well marked in proportion to the muscu- 



