KEGENERATION OF BONE. 371 



of new liver-cells and bile-ducts after injury to the liver ( 173), and Pisenti makes 

 the same statement as regards the kidney. After mechanical injury to the secre- 

 tory cells of glands (liver, kidney, salivary, Meibomian), neighbouring cells undergo 

 proliferation aud aid in the restoration of the cells. 



7. Amongst connective tissues, cartilage, provided its perichondrium be* not 

 injured, reproduces itself by division of its cartilage cells ; but usually when a part 

 of a cartilage is removed, it is replaced by connective-tissue. 



8. When a tendon is divided, proliferation of the tendon cells occurs, and the 

 cut ends are united by connective-tissue. 



9. The reproduction of bone takes place to a great extent under certain conditions. 

 If the articular end be removed by excision, it may be reproduced, although there 

 is a considerable degree of shortening. Pieces of bone which have been broken off 

 or sawn off heal again, and become united with the original bone. A tooth may be 

 removed, replanted in the alveolus, and become fixed there. If a piece of periosteum 

 be transplanted to another region of the body, it eventually gives rise to the forma- 

 tion of new bone in that locality. If part of a bone be removed, provided the 

 periosteum be left, new bone is rapidly reproduced ; hence, the surgeon takes great 

 care to preserve the periosteum intact in all operations where he wishes new bone 

 to be reproduced. Even the marrow of bone, when it is transplanted, gives rise 

 to the formation of bone. This is due to the osteoblasts adhering to the osseous 

 tissue. 



In fracture of a long bone, the periosteum deposits on the surface of the ends of the broken 

 bones a ring of substance which forms a temporary support, the external callus. At first this 

 callus is jelly-like, soft, and contains many corpuscles, but afterwards it becomes more solid 

 and somewhat like cartilage. A similar condition occurs within the bone, where an internal 

 callus is formed. The formation of this temporary callus is due to an inflammatory prolifera- 

 tion of the connective-tissue corpuscles, and partly to the osteoblasts of the periosteum and 

 marrow. According to Rigal and Vignal, the internal callus is always osseous, and is derived 

 from the marrow of the bone. The outer and inner callus become calcified and ultimately 

 ossified, whereby the broken ends are reunited. Towards the fortieth day, a thin layer of bone 

 is formed (intermediary callus) between the ends of the bone. Where this begins to be 

 definitely ossified, the outer and inner callus begin to be absorbed, and ultimately the inter- 

 mediary callus has the same structure as the rest of the bone. 



There are many interesting observations connected with the growth and metabolism of bones. 

 1. The addition of a very small amount of phosphorus or arsenious acid to the food causes con- 

 siderable thickening of the bones. This seems to be due to the non-absorption of those parts of 

 the bones which are usually absorbed, while new growth is continually taking place. 2. When 

 food devoid of lime-salts is given to an animal, the growth of the bones is not arrested, but the 

 bones become thinner, whereby all parts, even the organic basis of the bone, undergo a uniform 

 diminution. 3. Feeding with madder makes the bones red, as the colouring matter is deposited 

 with the bone-salts in the bone, especially in the growing and last formed parts. In birds the 

 shell of the egg becomes coloured. 4. The continued use of lactic acid dissolves the bones. 

 The ash of bone is thereby diminished. If lime-salts be withheld at the same time, the effect 

 is greatly increased, so that the bones come to resemble rachitic bones. (Development of Bone, 

 447.) 



When a lost tissue is not replaced by the same kind of tissue, its place is always 

 taken by cicatricial connective-tissue. 



When this is the case, the part becomes inflamed and swollen, owing to an exudation of 

 plasma. The blood-vessels become dilated and congested, and, notwithstanding the slower 

 circulation, the amount of blood is greater. The blood-vessels are increased, owing to the 

 formation of new ones. Colourless blood-corpuscles pass out of the vessels and reproduce them- 

 selves, and many of them undergo fatty degeneration, whilst others take up nutriment and 

 become converted into large uninucleated protoplasma cells, from which giant cells are developed. 

 The newly formed blood-vessels supply all these elements with blood. 



245. TRANSPLANTATION OF TISSUES. The nose, ear, and even a finger, after having 

 been severed from the body by a clean cut, have, under certain circumstances, become united 

 to the part from which they were removed. The skin is frequently transplanted by surgeons, 

 as, for example, to form a new nose. The piece of skin is cut from the forehead or arm, to 

 which it is left attached by a bridge of skin, is then stitched to the part which it is desired to 

 cover in, and when it has become attached in its new situation, the bridge of skin is severed. 



