146 



NATURE 



[December 15, 1904 



gives the facts gathered and the conclusions reached 

 during a prolonged research into the development, 

 comparative anatomy, and nature of the human 

 sternum. Leaving aside the convenience of having 

 our scattered knowledge on this subject summarised, 

 and the value of the mass of evidence collected during 

 the examination of hundreds of individuals, the main 

 importance of the work lies in two conclusions which 

 Prof. Paterson draws concerning the nature of the 

 sternum : — (i) that it is fundamentally part of the 

 shoulder girdle ; (2) that it is not a segmental structure. 

 Both these inferences are at variance with accepted 

 opinion. 



At the present time it is universally taught that the 

 sternum in mammals, birds and reptiles — that is to say, 

 in all vertebrates which use the body wall for the pur- 

 poses of inspiration — is a composite bone derived from 

 a fusion of the ventral ends of the ribs. The sternum 

 is thus regarded as a structure of costal origin, and 

 having only a secondary connection with the shoulder 

 girdle. In Amphibia, on the other hand, it is recog- 

 nised that the sternum is developed in continuity with 

 the shoulder girdle, of which it forms an intrinsic part; 

 it is in them a shoulder-girdle sternum. That the 

 shoulder-girdle sternum represents the more primitive 

 type, and that from such a type the costal sternum of 

 the Reptilia was evolved, are assumptions which com- 

 parative anatomists will freely grant. At present, 

 however, there is a distinct break in our knowledge 

 of the history of the sternum; no intermediate forms 

 between those two types are believed to occur, and no 

 one, with perhaps the exception of the late Prof. T. J. 

 Parker, has ever formulated a definite theory as to the 

 manner in which the costal sternum of Reptilia could 

 have arisen from the amphibian shoulder-girdle 

 sternum. Prof. Paterson 's investigations help us very 

 materially to trace the origin of the costal or, as it 

 may more truly be named, the " respiratory " sternum 

 of the three higher classes of vertebrates from the 

 simple sternum of Amphibia. He shows that the 

 " respiratory " sternum arises developmentally in con- 

 tinuity with the precoracoid element of the shoulder- 

 girdle, and quite independently of the ribs, and that 

 it is therefore merely a modified form of the amphibian 

 shoulder-girdle sternum. Further, the various forms 

 assumed by the " respiratory " sternum in reptiles, 

 birds, and mammals do not, when rightly interpreted, 

 favour Gegenbaur's conception of its evolution by a 

 fusion of the ventral ends of ribs. The sternum of 

 amphibians is the median ventral element of their 

 shoulder girdle, and when Prof. Paterson states that 

 no corresponding element is developed elsewhere in 

 the median ventral line, he overlooks the cartilage 

 developed as a median ventral element in the pelvic 

 girdle which in every sense exactly corresponds to the 

 sternum. 



The origin of the " respiratory " sternum is part of 

 a wide problem, viz. in what manner and under what 

 conditions did the body wall become modified to serve 

 as an active inspiratory agent in higher verte- 

 brates, thus replacing the " pharyngeal pump " of 

 amphibians? Whatever may have been the exact 

 manner in which the one form of respiration was 

 NO. 1833, VOL. 71] 



evolved from the other, there can be no doubt that the 

 ribs, the intercostal muscles, and the sternum as we 

 know them in higher vertebrates appeared during this 

 phase of evolution. Their appearance is directly due 

 to the introduction of a new type of respiration ; the 

 sternum which serves in the higher forms as an 

 element of the respiratory thorax is totally unlike the 

 bone which merely served as part of the shoulder 

 girdle in the more primitive type. With this evidence 

 clearly in view it is difficult to understand how Prof. 

 Paterson concludes that even in mammals the sternun* 

 is still — what it was when it first appeared in verte- 

 brates — functionally and fundamentally an adjunct or, 

 element of the shoulder girdle. We are surprisingly 

 ignorant of the part played by the sternum in the move- 

 ments of respiration, even in man, but a cursory ex- 

 amination of its respiratory movements in various 

 groups of birds, and in several orders of mammals, 

 I quickly serves to show that its form and size depend 

 chiefly not on the movements of the forelimbs, but on 

 the part it plays in the respiratory movements of the 

 thorax. In our opinion the key to the morphology of 

 the sternum is an accurate investigation of its function. 



Prof. Paterson is undoubtedly right in regarding the 

 sternum as primarily a continuous unsegmented 

 median bar. The conception of the sternum as a 

 segmental structure he characterises as " a nebulous 

 transcendental notion." Yet his own evidence shows 

 that the greater part of the mammalian sternum, at 

 the commencement of the cartilaginous and osseous 

 stages of development, is laid down as a truly 

 segmental structure, each segment corresponding 

 exactly to a body segment. Much more " nebulous 

 and transcendental " appears to us his explanation of 

 the occurrence of bony segments or sternabrae as " due 

 to the traction or pressure on the part of the ribs and 

 costal cartilages." In support of this theory Prof. 

 Paterson cites the fact that centres of ossification 

 appear in bones at points of traction and pressure. In 

 the case of the sternum, however, the centres of ossifi- 

 cation appear not opposite such points, but exactly 

 between them. 



This monograph is well got up; the figures are 

 numerous and highly finished. There is evidently a 

 slight error in Fig. 35, plate v.; the centre of ossifi- 

 cation for the fourth segment (if the term may still 

 be used) of the mesosternum is stated to be present in 

 71 per cent, of cases, whereas in the text (p. 18) the 

 proportion is given as 26 per cent. A curious misprint 

 occurs on p. 33, where the centre just alluded to is 

 said to appear in 59 per cent, of children before birth, 

 and 15 per cent. a\ter deafh— probably meaning after 

 birth. 



(3) The brothers Weber were of opinion that in the 

 forward swing of the leg in walking the lower ex- 

 tremity acted as a pendulum, the chief force in action 

 being that of gravity. Duchenne, on the other hand, 

 as the result of a special investigation, came to a totally 

 different conclusion, viz. that the forward swing was 

 almost wholly due to the direct action of muscle. In 

 the fifth and sixth parts of his research into the 

 mechanics of the human gait, Prof. Fischer concludes, 

 after an elaborate analysis of the force expended during 



