HYGIENE OF GROWING SKELETON. 109 



we find them all represented merely by cells with struc- 

 tureless intercellular substance : a little higher in the scale 

 the latter becomes fibrillated and forms distinct connective 

 tissue. In the highest Mollusks (see Zoology), as the cuttle- 

 fishes, this is partly replaced by cartilage, and the same 

 is true of the lowest fishes; while in some other fishes and 

 the remaining Vertebrates, we find more or less bone ap- 

 pearing in place of the original connective tissue or carti- 

 lage. 



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 all three (bone, cartilage, and 

 connective tissue) together as homologous tissues and re- 

 gard them as differentiations of the same original struc- 

 ture. 



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 

 Luge 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. XX. ), and no other food can thoroughly 

 replace it. If the mother's health be such as to render it 

 unwise for her to nurse her infant, the best substitute, 

 apart from a wet-nurse, will be cow's milk diluted with one 

 fourth its volume of water. Arrowroot, corn-flour, and 

 other starchy foods will not do alone, since they are all defi- 

 cient in the required salts, and many infants though given 

 food abundant in quantity are really starved, since their food 

 does not contain the substances 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 cartilagi- 

 nous layer, and at those points the bone increases in length, 

 new cartilage being formed and replaced by it. The bone 



