8 OSTEOLOGY. 



animal constituent or organised basis of the bone-tissue, "which retains 

 not only the general form, but also the minute structure of the original 

 bone. This is a tough, flexible, and mainly fibrous substance, "which is 

 capable of being in great part resolved into gelatin by boiling. 



Tlie ends of the bones, when jointed moveably ^Yith others, are covered 

 by a thin layer of dense permanent cartilage, called articular cartilage ; 

 and the adjacent bones are united together by fibrous ligaments which 

 may be considered as continuous with the periosteum covering the rest 

 of the bones. In some instances distinct bones are directly united by 

 means of ligament or cartilage without any joint-cavity intervening. 

 Thus the osseous system as a whole may be considered to be enveloped 

 by a fibrous covering. 



The bones are originally formed by a process termed ossification from 

 soft substance. This })rocess commences in the greater number of bones 

 in cartilage ; in some it begins in fibrous tissue or membrane ; and in all 

 instances the further growth of the bone substance takes place largely 

 in the latter way. The deposit of bone begins generally at one spot, 

 which is tlierefore called the original point or primary centre of ossifica- 

 tion ; bat there are sometimes several of these from the first. In most 

 bones, after considerable advance in growth by extension from the 

 original centre, ossification occurs at comparatively later periods in one 

 or more separate points, forming secondary or tertiary centres ; and the 

 portions of bones so formed, and remaining united to the main part for 

 a time by intervening cartilage, are termed epiphyses. In many instances 

 entire consolidation of the bone by the osseous union of the epiphyses 

 does not take place till the full size has been attained, and this may be 

 as late as the twenty-third or even the twenty-fifth year of life. 



In their outward form the bones present much diversity, but have 

 been reduced by auatomists to the following classes: — 1. Long or 

 cylindrical, such as the chief bones of the limbs. These consist of a 

 body or shaft, cylindrical or prismatic in shape, and two extremities 

 which are usually thicker than the shaft, and have smooth cartila- 

 ginous surfaces for articulation with neighbouring bones. The shaft is 

 generally hollow and filled with marrow, by which sufficient size and 

 strength are attained without undue increase of weight. 2. Tabular 

 or flat bones, like the scapula, ilium, and the bones forming the roof 

 and sides of the skull. 3. Short bones, which are more or less rounded 

 or angular, as in the carpus and tarsus. 4. Irregular or mixed bones, 

 mostly situated symmetrically across the median plane of the body, and 

 often of a complex figure, such as tlie vertebras. 



In these differently shaped bones the osseous substance occurs in two forms, 

 Tiz., the compact and the spong-y. There is, however, no essential difference in 

 stiTicture or properties between these beyond that of thickness or thinness of the 

 component material. 



The surfaces of bones present vai-ious eminences, depressions, and other marks, 

 to designate which the foUowmg tei-ms are in common use. Any marked bony 

 2irominence is called a process or ajwjthyxl^, the main part of the bone being 

 sometimes named diaphij.sis ; while processes originally ossified from a distinct 

 centre are during their separate condition named rj?ij)hi/,i/:s\ A slender, sharp, or 

 pointed eminence is np.med a .spine, or .y/iiioii.s jjvocoih ; a blunt one a tubercle ; a 

 broad and rough one a tvhcroxtti/. The terms crext. line, and 7'i(lf/e are usually 

 applied to a prominent border, or to an elevation running some way along the 

 surface of a bone. A fiead (caput, capitulum, or capitellum) is a rounded process 

 supported on a narrower paii- named the nccli (cervix). The term condi/lc, some- 



