> 



28 THE SKELETON 



young bones, at the epiphysial lines, i. e., the lines of junction of the main part of a bone with the 

 extremities or epiphyses. Long bones increase in length at the epiphysial cartilages, and increase 

 in thickness by ossification of the deeper layers of the investing membrane or periosteum. 

 These processes — intracartilaginous and intramembranous ossification — proceed concurrently 

 in the limb-bones of a young and growing mammal. 



There is no bone in the human skeleton which, though pre-formed in cartilage, is perfected 

 in this tissue. The ossification is completed in membrane. On the other hand, there are nu- 

 merous instances in the skull, of bones the ossification of which begins in, and is perfected by, the 

 intramembranous method. Ossification in a few instances commences in membrane, but later 

 invades tracts of cartilage; occasionally the process begins in the perichondrium and remains 

 restricted to it, never invading the underlying cartilage, which gradually disappears as the result 

 of continued pressure exerted upon it by the growing bone. The vomer and nasal bones are 

 the best examples of this mode of development. Further details of development and ossification 

 are included in the description of each bone. 



The limb-bones differ in several important particulars from those of the skull. Some of 

 the long bones have many centres of ossification, but these have not the same significance as 

 those oif the skull. It is convenient to group the centres into two sets, primary and secondary. 

 The primary nucleus of a long bone appears quite early in foetal life, and the main part (shaft) 

 thus formed is called the diaphysis. In only three instances does a secondary centre appear 

 before birth, e. g., the lower end of the femur, the head of the tibia, and occasionally the head of 

 the humerus. Many primary ossific nuclei appear after birth, e. g., those for the carpal bones, the 

 cuneiform and navicular bones of the foot, the coracoid process of the scapula, and for the third, 

 fourth, and fifth pieces of the sternum. 



When a bone ossifies from one nucleus only, this nucleus may appear before or after birth. 

 Examples: the talus (astragalus) at the seventh month of foetal life, and the lesser multangular 

 (trapezoid) at the eighth year. When a bone possesses one or more secondary centres, the pri- 

 mary nucleus, as a rule, appears early. Examples: the femur, humerus, phalanges, and the 

 calcaneus. 



Secondary centres which remain for a time distinct from the main portion of a bone are 

 termed epiphyses. An epiphysis may arise from a single nucleus, as is the case at the lower end 

 of the femur, or from several, as at the upper end of the humerus. Prominences about the ends 

 of long laones may be capped by separate epiphyses, as illustrated at the upper end of the femur. 



According to Professor F. G. Parsons, there are at least three kinds of epiphyses: — (1) 

 Those which appear at the articular ends of long bones, which, since they transmit the weight 

 of the body from bone to bone, may be termed pressure epiphyses. (2) Those which appear as 

 knob-like processes, where important muscles are attached to bones; and as these are concerned 

 with the pull of muscles, they may be described as traction epiphyses. (3) The third kind 

 includes those epiphyses which represent parts of the skeleton at one time of functional import- 

 ance but which, having lost their function, have now become fused with neighbouring bones and 

 only appear as separate ossifications in early life. These may be termed atavistic epiphyses 

 and include such epiphyses as the tuberosity of the ischium, the representative of the 

 hypoischium of reptiles. 



The epiphyses of bones seem to follow certain rules, thus: — 



1. Those epiphyses whose centres of ossification appear last are the first to unite with the 

 shaft. There is one exception, however, to this statement, viz., the upper end of the fibula, 

 which is the last to unite with the shaft, although its centre appears two years after that for the 

 lower end. This may perhaps be accounted for by the rudimentary nature of the proximal end 

 of the fibula in man and many other mammals. 



2. The epiphysis toward which the nutrient artery is directed is the first to be united with 

 the shaft. It is also found that while the increase in length of the long bones takes place at the 

 epiphysial cartilages, the growth takes place more rapidly and is continued for a longer period at 

 the end where the epiphysis is the last to unite. It follows, therefore, that the shifting of the 

 investing periosteum, which results from these two factors, leads to obliquity of the vascular 

 canal by drawing the proximal portion of the nutrient artery toward the more rapidly growing 

 end. Moreover, when a bone has only one epiphysis, the nutrient artery will be directed toward 

 the extremity which has no epiphysis. 



'.i. The centres of ossification appear earliest in those epiphyses which bear the largest 

 relative proportion to the shafts of the bones to which they belong. 



4. When an epiphysis ossifies from more than one centre, the various nuclei coalesce before 

 the shaft and ('i)iphysis consolidate, e. g., the upi)er end of the humerus. 



On section, the shaft of a foetal long bone is seen to be o(Hnipied by red marrow lodged in 

 bony cells which do not present any definite arrangement. In an adult the central portion of 

 the shaft is filled with fat or marrow lu^ld togetlior by a delicate reticulum of connective tissue, 

 whfrnce the space is known as the medullary cavity. The expanded end.s of the bones contain a 

 network of cancellous tissue, the intervals being filled with red marrow. This cancellous tissue 

 difTfrs from that of the fcjctal bone in l)eing arranged in a definite manner according to the direc- 

 tion of pressure exerted by the weight of the body, and the tension ))roduced by the muscles. 

 The arrangciiicrit of the caiuicUi in (lonsequence of the mechanical condit ions to which bones are 

 subject is noticed in the description of a vertebra, th(> femur, and the humerus. 



iiones are divisible! into four classes: — long, short, flat, and irregular. The long bones, 

 found chiefly in the limbs, form a systcim of levers sustaining the weight of the trunk and i)rovid- 

 ing the means of locomotion. The short bones, illustrated by those of the carpus and tarsus, 

 are found mainly where compactness, elasticiity, and limited motion are the principal require- 

 ments. Flat bones confer protection or provide broad surfaces for nuisciilar attachment, as 

 in the case of th(! cranial bones ancl the shoulder-blade. Lastly, the irregular or mixed bones 

 constitute a group of peculiar form, often very comi)lex, which cannot be included under either 

 of the preceding heads. Thc^e are the vertebrie, sacrum, coccyx, and many of the bones of the 

 skull. 



