106 THE HUMAN BODY. 



not only represent one another at different stages in the life 

 of the same animal but permanently throughout the whole 

 life of different animals. Low in the animal scale we find 

 them all represented merely by cells with structureless inter- 

 cellular substance: a little higher in the scale the latter 

 becomes fibrillated and forms distinct connective tissue. 

 In the highest Mollusks (as the cuttle-fishes) this is 

 partly replaced by cartilage, and the same is true of the low- 

 est fishes; while in some other fishes and the remaining 

 Vertebrates we find more or less bone appearing in place of 

 the original connective tissue or cartilage. 



From the similarity of their modes of development and 

 fundamental structure, the transitional forms which exist 

 between them, and the frequency with which they replace 

 one another, histologists class the three (bone, cartilage, and 

 connective tissue) together as homologous tissues and regard 

 them as differentiations of the same original structure. 



Hygienic Remarks. Since in the new-born infant many 

 parts which will ultimately become bone consist only of car- 

 tilage, the young child requires food which shall contain a 

 large proportion of the lime-salts which are used in building 

 up bone. Nature provides this in the milk, which is rich in 

 such salts (see Chap. XXI), and no other food can thoroughly 

 replace it. Long after infancy milk should form a large 

 part of a child's diet. Many children though given food 

 abundant in quantity are really starved, since their food does 

 not contain in sufficient amount the mineral salts requisite 

 for their healthy development. 



At birth even those bones of a child which are most ossi- 

 fied are often not continuous masses of osseous tissue. In the 

 humerus, for example, the shaft of the bone is well ossified 

 and so is each end, but between the shafts and each of the 

 articular extremities there still remains a cartilaginous layer, 

 and at those points the bone increases in length, new cartilage 

 being formed and replaced by bone. The bone increases in 

 thickness by new osseous tissue formed beneath the perios- 

 teum. The same thing is true of the bones of the leg. On 

 account of the largely cartilaginous and imperfectly knit 

 state of its bones, it is cruel to encourage a young child to 

 walk beyond its strength, and may lead to "bow-legs" or 

 other permanent distortions. Nevertheless here as elsewhere 

 in the animal body, moderate exercise promotes the growth of 



