22 THE BONES. 



Toussaint has also studied the course of ossification in the Bog, Pig, and 

 Rabbit. In the Dog at birth, none of the complementary nuclei have been 

 invaded by ossification. Consequently, bone tissue is only found in the dia- 

 physis of the long and flat bones. In the thrge months after birth, the great 

 majority of the epiphyses commence to ossify ; and after this period there only 

 remain the pyramidal bone and the complementary nuclei of the coxffi, in the 

 cartilaginous condition ; the ossification of these latter commence at from five to 

 six months. 



The skeleton of the Rabbit is, at birth, in a condition almost identical with 

 that of the Dog ; nearly all the epiphysary nuclei of the humerus and that of 

 the lower end of the femur, however, show osseous points. 



In the Fig, at birth ossification is much more advanced than in the Dog ; 

 but it is less so than in the larger Herbivora. 



The fusion of the epiphyses has been studied in the Dog. It commences at 

 five months, in the phalanges and metacarpals, and continues in the humerus 

 and radius, the corresponding epiphyses of which are consolidated at nine 

 months ; at eighteen months there can be distinguished the nuclei at the upper 

 border of the scapula, the upper end of the humerus, inferior extremity of the 

 radius and uhia, the epiphyses of the femur, and the bodies of the vertebrae. 

 The latter facts support the preceding ones, and likewise demonstrate that the 

 conclusions of Sanson are probably too absolute. 



Growth of the Bones. 



Bones increase by the superposition of new elements, while the soft parts of 

 the organism grow by the interposition of new elements in the mass of pre- 

 existing elements. The manner in which this apposition of new elements is 

 accomplished is not the same in the long, the flat, or the short bones. 



1. Long Bones. As a general rule, the long bones elongate by the growth 

 and ossification of the temporary cartilage situated between the diaphysis and 

 the osseous nuclei at the extremities. Consequently, elongation ceases when the 

 primitive or complementary nuclei are fused with one another. The increase in 

 length in the bones of the Ihnbs does not take place everywhere in the same 

 proportion. Duhamel, Flourens, and particularly Oilier and Humphry, have 

 remarked that, in the thoracic limb, the extremity furthest removed from the 

 humero-radial articulation grows fastest ; while in the abdominal limb, the 

 extremity most distant from the femoro-tibial articulation grows the least. 



With regard to the increase in thickness of the bone, this occurs by ossifica- 

 tion of the deeper layer of the periosteum — the osteogenous lager. The experiments 

 of the authors just mentioned irrefutably demonstrate this, and those of Oilier 

 have even proved that the periosteum may produce bone at a point where it has 

 been transplanted. 



The periostic bone is developed according to the process of ossification in the 

 fibrous tissue. In a transverse section of the diaphysis of a growing long bone, 

 there is seen, as Laulaine has indicated, concentric circumferences united by 

 radii of osseous substance. The diaphysary portion of the periostic bone is 

 fused with the extremities, through the medium of the ossification notch of 

 Ranvier — a prolongation of the periosteum into the articular enlargement around 

 the cartilage of conjugation. The formation of bone tissue in the deeper layer 

 of the periosteum is very active during the youth of animals, but it soon 

 diminishes, and ceases completely in advanced age. 



