Maech 39, 1907] 



SCIENCE 



493 



liave seen, is far below the capacity of their 

 tissues, the distribution could occur only 

 in two ways. Either some parts of the tis- 

 sues work to their full capacity, while the 

 other parts remain idle, being only in 

 readiness for emergencies — like the unem- 

 ployed vice-president of some organization 

 — or all elements of the organ take equal 

 part in the work, each tissue element em- 

 ploying only a fraction of its capacity for 

 work. The last alternative is probably the 

 more frequent mode of distribution. There 

 are, for instance, probably no totally inac- 

 tive glomeruli and tubules in the kidneys, 

 no inactive liver cells, no thyroid epithelial 

 cells entirely without colloidal substance, 

 but the epithelium of the glomeruli and 

 tubules work only one half of their ca- 

 pacity, the islands of Langerhans work 

 less than one tenth, the vesicles of the 

 thyroid about one sixth of their capacity, 

 etc. For the muscles of the heart it is gen- 

 erally assumed that all the fibers take part 

 in every contraction, but that they work 

 normally only a fraction of their capacity. 

 On the other hand, there are organs in 

 which surely parts of the tissue do not take 

 active share in the work, unless called 

 upon under special circumstances. In the 

 ovaries, for instance, surely only one ovum 

 becomes fertilized, while all the others are 

 only on the waiting list. An instructive 

 instance is the mode of distribution of 

 work among the respiratory muscles. In 

 normal inspirations, for instance, we find 

 only the diaphragm alone at work. When 

 somewhat deeper breathing is required, the 

 inspirations are supported by the levatores 

 costarum and the scaleni. Furthermore, 

 in labored respirations also the sternohyoid 

 and the posterior superior serrati become 

 engaged in the work, and when the diffi- 

 culties become still greater, still other 

 groups of muscles enter into the struggle, 

 in other words, the different groups of 



muscles which are designated to do the 

 work of inspiration are not engaged in it 

 in the manner of partners of equal stand- 

 ing, but enter upon their duties as a series 

 of vice-presidents or rather as a series of 

 reserve forces. On the other hand, in the 

 diaphragm probably all the muscle fibers 

 are engaged in the work of each inspiration 

 at all times, employing only a fraction of 

 their capacity in normal or shallow inspira- 

 tion and working to their utmost capacity 

 in dyspnoea or asphyxia. We see, there- 

 fore, in one and the same function both 

 modes of distribution of work well repre- 

 sented, one muscle steadily at work with all 

 fibers, like a heart, adapting the degrees of 

 their energies to the various requirements 

 of their work, and a number of groups of 

 other muscles, acting as graded reserve 

 forces, idle but ready for emergencies — 

 instructive examples of luxurious factors 

 of safety. 



In the foregoing we have brought for- 

 ward a sufficient number of instances in 

 which various parts of the living organism 

 are provided with a superabundance of 

 material and energy to warrant the com- 

 parison of the organism with a machine 

 with regard to the provision with factors 

 of safety. 



One of the fundamental differences be- 

 tween living organisms and human-made 

 machines is that the former carries in it 

 the germ for self-propagation, while ma- 

 chines have to be made by human hands. 

 As a further difference between the two 

 constructions we may perhaps consider the 

 phenomenon of self-repair. Possibly the 

 phenomenon of self-repair in the organism 

 is closely allied with the phenomenon of 

 self -propagation. The same source which 

 provides the organism with a mechanism 

 for a reproduction of the entire body pro- 

 vides its parts with a mechanism for regen- 

 eration of these parts. Eeproduction and 



