GENERAL CONSIDERATION OF THE JOINTS. 107 



lescence. He is convinced, as his statements will show, that this union occurs earlier 

 than is generally taught. 



Relation of the Bones to the Figure. — While it may be said that power- 

 ful muscles leave their imprint on the bones in strong, rough ridges, yet it is impos 

 sible to give a trustworthy description of the figure from the size and shape of the 

 bones, since these are determined chiefly by prenatal influences. Very delicate, 

 even puny, bodies may have large and strong bones, and great muscular develop- 

 ment may coexist with a light framework. 



Variations. — Besides the great range of individual variation, without departure 

 from the usual type, bones occasionally show greater peculiarities. These may occur 

 through either excess or defect of ossification. Structures which are normally car- 

 tilaginous or fibrous may become replaced by bone, and abnormal foramina may 

 occur in consequence, or to accommodate the aberrant course of blood-vessels or 

 nerves. The most interesting of these variations are such as present an arrangement 

 which is normal in some of the lower animals. Many variations may be plausibly 

 accounted for as reversions, but others cannot be explained in this way according 

 to any conceivable scheme of descent. By speaking of these variations as animal 

 analogies we avoid theories and keep to scientific truth. 



Number of Bones. — The usual enumeration of the bones composing the 

 human skeleton is misleading, for while it is customary in some parts, as the head, 

 to count each bone, in others, like the sacrum and the hyoid, only the ultimate 

 condition, after union of the component segments, is considered. In other cases, 

 like the sternum, there may be grave doubt which course is the proper one to 

 follow ; and finally, as in the coccyx, the number is variable. Bearing these impor- 

 tant facts in mind, it may be stated that the human skeleton in middle life usually 

 comprises, as conventionally reckoned, two hundred separate bones, excluding the 

 sesamoids within the tendons of the short flexor of the thumb and of the great toe 

 and the ear-ossicles, but including the patella and the hyoid bone. Of this number, 

 seventy-four bones belong to the axial and one hundred and twenty-six to the appen- 

 dicular skeleton. 



The skeleton is advantageously described in the following order : the spine, the 

 thorax, the head, the shoulder-girdle and the arm, the pelvic girdle and the leg. 

 The account of the bones of each region is succeeded by that of the joints and the 

 ligaments holding th^m together, followed by a consideration of the region as a 

 whole and of its relation to the surface. The applications of anatomical details of 

 the skeleton to the requirements of medicine and surgery are pointed out in appro- 

 priate places. 



GENERAL CONSIDERATION OF THE JOINTS. 



A JOINT or articulation implies the union of two or more bones. Joints may be 

 divided, according to their mobility, into three great classes : the fixed joint (Syn- 

 arthrosis'), the half-joint {Amphiarthrosis), and the true joint {Diarthrosis). 



Fixed Joints. — These allow no motion in the mature condition, and are rep- 

 resented by two subdivisions, the Suture and the Syyichondrosis . 



The suture is the direct union of two bones which at first may be separated by 

 membrane or by fibrous tissue, but which eventually become firmly united. Several 

 varieties of this form of union are recognized ; thus a serrated suture is one in which 

 the edges are interlocked, as the teeth of two saws ; conspicuous examples are seen 

 in the interparietal and the parieto-occipital junctures. Frequently one bone tends 

 to overlap at one end of the suture and to be overlapped at the other. A sqtiamous 

 stiture is one in which a scale-like bone very much overlaps another, as in the relation 

 between the temporal and the parietal bone. An harmonic suture is one in which 

 two approximately plane surfaces are apposed, as in the case of the vertical plate of 

 the palate and the maxillary bone. The term grooved suture is sometimes employed 

 to designate a form of union in which one bone is received within the grooved sur- 

 face of another, as the rostrum of the sphenoid and the vomer. Wormian boyies are 

 small, irregular ossifications which appear as bony islands in the course of a suture. 

 Familiar examples of these are seen in the line of the parieto-occipital suture. 



