1921 Brain  
THE factors that determine the structures in the nervous system, the form of the neurones, their position and connections, have puzzled the minds of many neurologists. This matter was first considered more thoroughly by embryologists, who, however, were chiefly interested in the problem of the connections between the central nervous system and the peripheral organs, the formation of nerve-roots. One of those was Hensen, who assumed " that all nerves originated by an insufficient separation in
more » ... ent separation in the primary connections between ganglion cells and their peripheral organs during evolution." Hensen was thus a forerunner of the conception that a real separation of the cellular constituents of the body never occurs, but that its elements remain connected with each other. A similar conception of syndesmism was defended in England by Sedgwick, who stated that " Nerves are developments of the reticulum, elongated strands of the pale substance composing this reticulum, with some of their nuclei." I shall come back later to this conception, which, although it may contain a true prinoiple, can never explain the peculiar selectivity in the neuronal connections. An opposite opinion was held by Balfour and His, who showed that the connections of the nerves with their end-organs were secondarily acquired. Of these two leading embryologists, His was generally inclined to regard mechanical factors as the most important in the embryonic evolution, and he practically saw in the growth and arrangement of the nervous elements a purely mechanical problem. He attempted to solve this problem by assuming that the direction of the nervous offshoots was determined by the places or paths of least resistance. The Belgian histologist Dustin a has pointed out that by such ^/reformed fwepaths we are able to understand why the regeneration after cutting a peripheral nerve runs so smoothly, guided by the degenerated nerve-sheaths of the peripheral stump; whereas in the central nervous system, which does not exhibit any sheaths of Schwann, regenerating nerve-fibres rarely reaoh 1 A lecture read in the University of London. ' Who termed this the hodogenetic principle. by guest on October 27, 2014 Downloaded from
doi:10.1093/brain/44.2.125 fatcat:27f7np3tazf75io7ywarwuximi