THE HUMAN STRUCTURE 99 



that is to say, a modulus of 146 and a force of cohesion of 7 kilo- 

 grammes on the average. Triepel finds R = about 5, a resist- 

 ance to traction equal to at least half that of the bones. 



The cartilages cover the bony joints and dominate the 

 skeleton of the young. They are more of a cellular than a fibrous 

 nature, and their proportion of water, which varies with age and 

 from one cartilage to another, is very high : 74% approximately 

 in the cartilage of the knee, 68% in the cartilages of the ribs. The 

 mineral salts which they contain, double in quantity between 

 the ages of 20 and 40. 



The resistance of cartilages to fracture is greater in compression 

 than in traction ; T5 kg. as against 0'18 kg., according to 

 Triepel. Young's modulus is, on the average, T50, almost 

 equal to that of the muscles, but it is rarely constant, for it can 

 attain a value of from 16 to 20 in particular cases. 



72. (iv). The Nerves. The nerves are white filaments which, 

 radiating towards all the points of the organism, carry there 

 the " motive excitement " or gather there the sensitive impres- 

 sions, the " sensation." They emanate from determined centres, 

 these centres being cellular, but the " nervous cell " emits a very 

 long prolongation to the centrifugal function., the cylindric axis, 

 and short appendices, the " dendrites," which connect it with 

 the neighbouring cells. The " spinal cord," which runs through 

 the spinal column, is a very important centre. 



Nervous matter contains from 62 to 80% of water, and its 

 density varies from 1 - 03 to T04 and slightly between one sex 

 and the other, f 1 ) 



The nervous cell, round or oval, is from -^ to x -|- of a milli- 

 metre in diameter ; it, and its prolongations, constitute together 

 a " neuron " ; the longest forming a fibre, and a fasciculus or 

 bundle of fibres are found in the single nerve, each having a 

 thickness of from 1 to 20 thousandths of a millimetre. 



The nervous cords take the name of the part to which they 

 are connected ; they distribute themselves in an unequal manner 

 and contain an unequal number of fibres. In the motor fibres, 

 for example, there are two or more per long striated muscular 

 fibre, while there is only one in a short fibre. This distribution 

 satisfies the functions of the muscles, which are so diverse and 

 unequal. Krause found 15,000 nervous fibres in the " motor 

 oculis " muscle, the six muscles of the eye together having nearly 

 18,000, and yet only weighing about 3 grammes. This proportion 

 would lead to 180 million nervous fibres in the whole muscular 

 system (weighing 30 kilogrammes), but according to Stirling there 

 are only 300,000 real motor fibres emanating from the spinal 

 column. 



l ) Gomperz, Journal of Physiol., vol. xxvii., p. 459 (1901-1902).. 



