216 SECTIONAL ADDRESSES. 



imprisonment, multiply. At first the force of gravity and the twitching 

 of muscles acting on the soft semi-fluid tissues between the broken ends 

 of the bone supply stimuli that are indeterminate in direction, and such 

 reaction as occurs results only in the formation of loosely ordered 

 calcareous fibres ; but even this soft callus gives some degree of rigidity, 

 sufficient to restrict the strains gradually to more and more clearly defined 

 lines along which in proportion a stronger reaction can take place. Once 

 it is established that bone corpuscles react to strain and stress by dis- 

 charging collagen, the intimate spatial disposition of which, as well as of 

 the lime-salts with which it comes to be encrusted, is determined by the 

 directing forces to which it is expoeed, and once it is recognised that 

 the law of spontaneous regeneration requires that this reaction will 

 persist in proportion to the prevalence of these forces, not only must the 

 gradual replacement of callus by appropriate permanent bone necessarily 

 follow, bone in which no particle persists except it be in the line of 

 constantly recurring stress and strain, but it will also necessarily follow 

 that the position of every spicule of bone in the skeleton, cancellous or 

 compact, is the expression of a physiological reaction to the forces of 

 gravity and muscular tension. The evolution of the machinery of the 

 connective tissues seems to be not entirely the result of natural selection 

 and the survival of individuals in which this machinery chanced to be of 

 appropriate design. The appearance in early vertebrates of the material 

 that is characteristic of the bone corpuscle seems to have ensured that 

 skeletons would take a shape determined by the direction of the forces 

 to which these corpuscles were exposed, and that the formation of this 

 skeleton is as much a reaction to recurring stimuli as are the reflexes, 

 composite movements, and postures characteristic for the species. 



This conception of the way in which the vertebrate connective tissues 

 take their shape transfers a large share of the development of the bodily 

 form back into the nervous system, in which the machinery is stored that 

 directs and determines the habitual movements and postures that in reac- 

 tion to external disturbances are specific. A physiological account of the 

 evolution of the nervous system, one certainly that is based on the chemical 

 constitution and chemical behaviour of its component parts, must seem 

 almost infinitely remote from practical investigation. But the work of 

 Paylov has made one thing clear, that by a physiological reaction in it 

 machinery may come into existence which did not exist before. The 

 repeated occurrence of a disturbance at times that are uniformly related 

 to the normal operation of existing machinery results in the acquirement 

 of a new reaction which must require machinery that is new. It is rendered 

 probable, if not proved, that this new machinery is situated in what may 

 be called the growing point of the central nervous system, the cortex of 

 the cerebral hemispheres, the part where all is not cut and dry, where 

 cells retain more of the properties of the developing neuroblasts, the 

 properties that enable them to grow out through the embryonic tissues 

 along courses that make it certain that the maturing organism will behave 

 in a manner true to type. In the formation of a conditioned reflex two 

 events are made to occur in the cerebral cortex at times which are uni- 

 formly related to one another ; one of these events, from the constitution 

 of the nervous system, necessarily results in a certain activity of some 



