JuLy 23, 1914] 
NATURE 
ou 
The Statesman’s Year-Book: Statistical and His- 
torical Annual of the States of the World for 
the Year 1914. Edited by Dr. J. Scott Keltie, 
assisted by Dr. M. Epstein. Fifty-first annuai 
publication. Revised after Official Returns. 
Pp. Ixxix+1500. (London: Macmillan and 
Comaora.) Price ros. 6d. net. 
As the years go by, the growth in size and usefui- 
ness of this welcome summary of the world are 
signs not only of the value of the contents, but 
of the carefulness which marks its compilation. 
Much wants more, and many readers would, no 
doubt, appreciate the extension of the introduc- 
tory tables to include world surveys of other com- 
modities than coal, gold, etc. The maps this year 
deal with new political boundaries in Balkania and 
Mongolia, the extension of railway communica- 
tions in America, and the position and number 
of the wireless stations of the world. Many por- 
tions of the main text have been subjected to a 
thorough revision by competent authorities, and 
no effort seems to have been spared to bring the 
fifty-first issue thoroughly up-to-date. The com- 
plete bibliographies add specially to the usefulness 
of this indispensable year-book. 
BELTERS TO THE EDIFOR. 
[The Editor does not hold himself responsible ‘for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications. ] 
Man’s Chin: a Dynamical Basis for Physical and 
Psycho-physiclogical Utilities. 
To account for the presence of man’s chin at least 
three different explanations have been brought forward 
and discussed :—(1) That the chin has been evolved 
by sex selection for its zsthetic value; (2) that it was 
needful for the development of the genio-glossal muscle 
and speech; (3) that with man’s erect posture the 
chin has been chiefly useful in affording room for 
important structures in the throat, and in protecting 
them during combat, etc. These explanations have so 
far met with very little acceptance. 
A conception of the chin as a dynamical factor in 
both mastication and speech does not appear to have 
received attention. An engineer examining the dental 
mechanism as a type of machine new to him would, 
on finding there was a considerable bulk of con- 
structional material projecting from the chief moving 
member, be nearly certain to ask—What does this do ? 
The chin mass is situated at the outer end of the jaw 
lever, where its momentum is greatest. It is built up 
in the heavier material used in the general construc- 
tion. There is another point, too, that one should not 
too readily dismiss as a mere coincidence. Every 
rotation movement of the mandible during its elevation 
or shutting has combined with it a movement— 
obliquely upward and backward—of translation. The 
combined movements are so directed that at some 
parts of the jaw the resultant velocity is less than 
would exist if either component were to act alone; and 
at about a point situated between the jaw angle and 
the condyle, the resultant velocity is so small that 
some observers mistakenly believed it to be nil. At 
the chin, on the other hand, the directions of the com- 
ponent movements are such that the resultant velocity 
reaches nearly its maximum acceleration. 2 
NO. 2334, VOL. 93] 
My suggestion is not quite that the chin is simply 
man’s masticating hammer; something rather less 
crude than a purely percussive function is condi- 
tioned by the momentum of the chin. No doubt the 
momentum of the chin may appear to be a very small 
contribution to the considerable muscular force often 
used in chewing. Yet on the teeth themselves many 
morphological details that have been preserved as dis- 
tinct specific features are so small that we do not yet 
know what the particular utilities are that determined 
their shape and survival. Further, there is another 
peculiarity in the mandible movement that may have 
some significance in this connection. During a (sup- 
posable) uniform movement of rotation about the 
condyle as horizontal axis, the accompanying trans- 
lation movement is not uniform, but relatively varied— 
slow or small in the beginning, quicker in the middle, 
and slower again towards the end of the condyle path. 
This is favourable to the normal rhythmical movement 
of the jaw by giving in some degree a pendulum-like 
character to its swing. And it so happens that the 
position of maximum velocity (and momentum) coin- 
cides with the position of greatest resistance and food- 
strain in chewing—that is, when the cutting-edges of 
the external blades of the lower cheek teeth are just 
about to pass their upper opponents in the inward-and- 
upward shearing thrust. The chin momentum 
operates most strongly just about the point where it 
is most useful in preserving the rhythmical movement 
of mastication, so as to render less necessary any 
consciously-directed variation in the muscular effort 
put forth in any single chewing stroke. 
Then, in the numerous smaller chewing movements 
for the finer reduction of food morsels, the chin mass 
(by both inertia and momentum) has at least some 
value as a ‘‘balance,’’ controlling and guiding the 
niceties of direction in the thrust. The utility of 
balance influences the construction of many man-made 
implements (pen- or brush-holder, razor handle, spear, 
etc.) in the use of which some precision is required ; 
this feature in construction has usually been adapted 
and has survived quite independently of any conscious 
or theoretical estimation of its special function. The 
obvious objection that animals manage the “niceties” 
of mastication without a chin could be met only by 
going more fully into the dynamics of the subject. This 
much at least can be stated here as being susceptible 
of proof—that as compared with the prognathous 
savage or the ape, the dental apparatus of modern 
civilised man is the ‘‘finer’’ machine, in so far as it 
is the better adapted for those shearing stresses by 
which tough foodstuffs are comminuted with economy 
of effort. 
The above suggestion of ‘‘balancing”’ and ‘‘ steady- 
ing ’’ utilities can also be applied to the rapid and yet 
delicately controlled movements of the mandible in 
speech. The man who wrote a book on ‘‘ The Speech 
of Monkeys”? might possibly have had hope of more 
success in interpretating the “language” of these 
animals if only he could have subdued and steadied 
their jibberings and chatterings by providing them 
with good weighty chins. D. M.) SHAW: 
Eltham, S.E. 
Meteoric Streaks and Trains. 
Pror. C. C. TRowBRIDGE, of New York, has been 
conducting an interesting investigation, during recent 
years, into the heights and velocities of the streaks 
and trains of meteors. He has been collecting old 
records of these phenomena, and will be glad to 
receive any new materials which may be gathered 
during this year’s Perseid shower. Every year brings 
us some brilliant Perseids leaving durable streaks, and 
it is important that when these appear the drift 
