Suppleme7it to "Nature,'' Attgust i8, 1923 



263 



and group themselves in a new way^ one which gave a 

 better functional result. 



Another striking fact is that the muscles which have 

 become reduced or vestigial in man have also become 

 reduced and vestigial, although usually to a less 

 extent, in the anthropoid apes. All of these muscles, 

 plantaris, palmaris longus, psoas parvus, latissimo- 

 condyloideus, omo-cervicalis, etc., are laid down in a 

 normal way during the development of the embryo ; 

 after being laid down retrogression sets in. We have 

 here again to deal with functional tendencies. The 

 machinery of reduction is resident in the processes 

 which govern the development of structural systems 

 in the embryo. As W. Roux supposed, there may be 

 a struggle for survival between the system of cells 

 which make up the body of an embryo. 



The Adaptational Properties of Bone Cells. 



By the fourth month of foetal life young nerve cells 

 and young muscle cells have taken up their definitive 

 position and arrangement. On the other hand, white 

 blood corpuscles retain all through the life of the 

 individual the migratory power which is lost by most 

 other cells of the body early in foetal life. The cells 

 which line blood and lymph vessels and those which 

 line the peritoneal and pleural cavities ^^ retain all 

 through life a power to proliferate and produce new 

 tissues which are of a purposive kind. Such cells 

 retain the chief characteristic of embryonic cells- — the 

 power to arrange themselves as part of a functional 

 complex. Bone cells also retain powers of purposive 

 action. Nothing is better known than that, if a bone 

 of a rickety child bends under the weight of the body, 

 the bone cells lying in its concavity will proliferate 

 and build a buttress to strengthen the shaft. It is 

 not necessary for us to speculate here as to the exact 

 stimuH which cause bone cells to behave in this manner ; 

 it is enough for our present purpose to note that they 

 react to fulfil an end necessary for the occasion. 20 



John Hunter discovered the remarkable power 

 which bone cells possess to remodel bones during 

 growth. While bone cells are building at one part 

 of a bone, they are, at another part of the same bone, 

 liusily engaged in taking down their previous handiwork. 

 The co-ordinated manoeuvres of the armies of bone 

 cells concerned in the growth of the jaws and eruption 

 of the teeth are extraordinary. When teeth are 

 erupting and also long after they are cut, their bony 

 sockets are being constantly altered and remodelled 

 by the hundreds of thousands of osteoblasts embedded 

 in the bone surrounding the dental roots. While 

 new bone is being laid down on the outer side of the 

 jaw under the gum, the corresponding l)one on the 

 inner side of the gum is being absorbed. But in the 

 tooth socket itself the opposite is happening ; new 

 l)onc is being laid down on the inner side of the tooth 

 socket, while it is being removed from the wall forming 

 the outer side of the socket. New bone is being laid 

 down under the roots so that the socket as a whole is 

 being raised and moved in an outward direction. 



•• An arrount of the actions and reactions of vascul.ir tissues will be 

 foiiiid in the writings of W. Roux from 1878 onwards. I have dealt with 

 the adaptativc reactions of peritoneal tells in Human Embryology and 

 Morphology, 1921, 4th edition. 



*" I have dealt with the growth reactions of bone cells at some length 

 (Menders of the Maimed, 1919, chapters xiv., xv., xvi., xvii., and xviii.) 



The crowd of osteoblasts involved in this operation 

 are clearly co-ordinated in their action ; they move 

 on towards a functional result. Although we do not 

 know the exact means by which their action is co- 

 ordinated we have, in the qualities and tendencies, 

 possessed by bone cells, part of the machinery of 

 evolution. Cartilage cells, during embryonic life, 

 must be co-ordinated in their growth and arrangement. 

 In the foetal hand we find they have fashioned the 

 joints to meet the needs of the muscles which act on 

 them, thus permitting harmonious movements of the 

 wrist and digits. The development and behaviour 

 of embryonic cartilage cells constitute part of the 

 machinery of human evolution. 



Co-ordination in the Growth of Bone, 

 Muscle, and Nerve Cells. 



I have dealt with the behaviour of young cells of 

 bone, muscle, and nerve centres in the developing 

 embryo in order that we may appreciate the com- 

 plexity of the process involved in producing a new 

 structural adaptation of the human body. When we 

 sit up or walk, our vertebrse are balanced one upon 

 another by means of a complex series of muscles acting 

 upon an equally complex series of levers, the whole 

 controlled by intricate groups of nerve cells situated 

 in the spinal cord and brain. The anatomical evid- 

 ence 2^ leaves us in no doubt that the spinal mechanism 

 of man has been evolved from one very similar to that 

 now seen in the anthropoid apes. Indeed in the 

 young chimpanzee and gorilla many of man's spinal 

 adaptations are already present. In the evolution of 

 a human from an anthropoid spine we have to conceive, 

 (i) that the multitudes of bone cells involved in the 

 building of vertebral processes of the embryo were so 

 influenced in their operations that the levers they 

 built were altered in strength, inchnation, and form ; 

 (2) the countless myriads of- myoblasts involved in 

 the formation of the spinal musculature were so 

 influenced that they took up new positions and effected 

 new combinations ; (3) the cartilage cells, which mould 

 the contours of the interverte])ral joints, were moved 

 to alter the shapes of the articular surfaces so as to 

 provide the needed contours ; (4) the nerve cells of 

 the spinal cord and brain, presiding over the reflex 

 and voluntary movements of the spinal muscles, had 

 to undergo increase in numbers, rearrangements in 

 grouping, and readjustment of contacts. We have to 

 postulate that in the human embryo there exists a 

 machinery which co-ordinates the development and 

 growth of all the diverse hordes of embryonic cells 

 concerned in the formation of man's spinal mechanism 

 and causes them to move in a direction which, at all 

 stages of evolution, yields a harmonious functional 

 result. 



Theory of Hormones. 



There is only one theory which affords a rational 

 explanation of how such complex adaptations can be 

 brought about — the theory of Hormones postulated 

 by Starling in 1905.22 Although Prof. Starling devoted 



" " Man's Posture : Its Evolution and Disorders," BrU. Med. Joum., 

 I923> I. PP- 451, 493. 545. 587. ''42, 66g. 



" Prof. E. H. .Starling, "The Chemirnl Correlation of the Functions of 

 the Body." The Croonian Leclims .il llie Roy.il College of Physicians^ 

 Lancet, 1905, vol. 2, p. 339. 



i^ 



