Osseous System. 157 



head and tail alternately towards one side and the other, which being 

 transferred in part to the water, ends in the progressive movement of 

 the fish. It is manifest that the first effect of every muscular tension 

 is a compression, or at least a compression strain, on some other part. 

 In the fish, even if it had no backbone, the compression strain would 

 necessarily come where the backbone is, since the tension being neces- 

 sarily bilateral, oscillatory and alternate, the muscles of the opposite 

 sides would alternately pull down upon that middle axis like playing at 

 see-saw. If that middle axis were muscle instead of bone, from its 

 very position it could never be called upon to perform the work of a 

 muscle, that is, contract, because its contraction could add nothing to 

 the locomotion of the fish. It would therefore remain rigid and func- 

 tionless as muscle, and, in time, lose its power of contraction. E very- 

 bod} 7 - knows the indurating effect of pressure upon the skin in the horny 

 hands of laborers, the soles of people who go bare-footed, the callosities 

 of monkeys and cavalry-men, and the thick necks of the working oxen. 

 Tissue formed under, pressure must of necessity differ in texture from 

 that thrown together without pressure. The difference between the 

 hard, compact muscles of a laboring man and the soft, flabby muscles 

 of the man who does nothing, is caused by the packing of man} r cells 

 of muscle tissue in a small space when exercise causes the frequent and 

 vigorous contractions of the muscle fibres. Every installment of blood 

 drawn by the irritation to the excited tract, deposits fresh particles of 

 nutrient matter, and every contraction of the muscle fibres packs them 

 down, as it were. Tendons and ligaments are bands of dense fibres of 

 that connective tissue which fills the spaces between the muscles and 

 surrounds the bones. There can be no reasonable doubt that the' con- 

 stant tension strain upon the tendons, coupled with the rigidity and fix- 

 edness of their position, has mechanically packed their fibres together. 



The ligaments are subjected to both tension and compression strains. 



The connective tissue also grades into gristle or cartilage possessing 

 some elasticity, and subjected chiefly to compression strains. Bone's are 

 formed by the deposit of phosphate and carbonate of lime among the 

 organic cells of cartilage, subverting the greater part of its elasticity 

 and giving greater density and rigidity to its structure. Cartilage is, 

 therefore, the incipient stage of bone. In the young or larval Ascidian, 

 the Amphioxus and the Selachian fishes (Sharks, Dog-fishes, &c.) the 

 osseous system remains in the cartilaginous stage. But in all the other 

 vertebrates most of the skeleton becomes ossified, although there re- 

 main in all of them some parts which never do. 



In the embryos of all vertebrates the skeleton begins as gelatinous 

 cartilage, and the ossification of the different parts takes place at differ- 

 ent times. At birth, the bones of human infants are soft and flexible; 



