THE HAND.--PROPORTIONS OF THE UPPER LIMB
The bony structure of the hand.--The wrist, or carpus; eight
bones in two rows, superior and inferior.--The radio-carpal
articulation, or wrist-joint.--Carpal joints.--The form of the
wrist during flexion.--The palm of the hand, or metacarpus;
metacarpal bones; their relative lengths (form of the
fist).--Carpo-metacarpal articulations; the articulation of
the thumb.--The fingers, or phalanges.--Articulations of the
phalanges; their movements.--Proportions of the upper limb: the
arms extended (the square figure of the ancients); the hand as
a common measure; the middle finger and the Egyptian Canon of
Charles Blanc.--Brachial index.
The hand is composed of three parts: the wrist, the palm, and the
fingers. The bony structure of the wrist is formed by the _carpus_,
that of the palm by the _metacarpus_ (μετὰ, below; καρπὸς, the wrist);
the fingers are formed by small bones called _phalanges_ (Fig. 25, page
80).
As the carpus is almost completely hidden by soft parts, fibrous and
tendinous, we shall first proceed to enumerate the bones which compose
it, and show their articulations.
Notwithstanding its small compass, the carpus is made up of not
less than eight bones, which are placed in two transverse rows, one
_superior_, or _brachial_ (next the forearm), the other _inferior_, or
_metacarpal_ (next the metacarpus).
The bones of the two rows are arranged as follows, enumerating them
in order from without inwards--that is, from the radial to the
ulnar border of the wrist:--The four bones of the first row are:
the _scaphoid_ (S, Fig. 28), named from the cavity on the inferior
surface being compared to a boat (_σκάφη_, a boat; _εἶδος_, form);
the _semi-lunar_ (L, Fig. 28); the _cuneiform_ (C, Fig. 28) (whose
names indicate their shape); and the _pisiform_ (P, Fig. 28), which,
small and rounded, is placed on the anterior surface of the cuneiform
bone, and articulates with it alone (Fig. 29). The four bones of the
second row, still naming them from without inwards, are (Fig. 28):
_trapezium_, _trapezoid_, _os magnum_, and _unciform_ bones (_uncus_, a
hook).
[Illustration: FIG. 28.
THE BONY STRUCTURE OF THE WRIST (dorsal surface): R, radius;--U,
ulna;--F, triangular fibro-cartilage;--S, scaphoid;--L,
semi-lunar;--C, cuneiform;--P, pisiform;--T, trapezium;--T,
trapezoid;--M, os magnum;--U, unciform.--Below the carpus: 1, 2,
3, 4, 5, the five metacarpal bones counting from that of thumb
(1).]
An examination of the bony structure of the carpus as a whole shows
that the anterior or palmar surface presents the form of a vertical
groove, limited on the inner side by the forward projections of the
pisiform and unciform bones, and on the outer side by the projections
of the scaphoid and trapezium. This groove is formed into a canal by
means of a broad fibrous band (the _anterior annular ligament_ of
the wrist), which passes like a bridge across the wrist between the
prominences just named. Beneath this bridge, and in the canal thus
formed, pass the tendons of the flexor muscles of the fingers, the
fleshy bellies of which occupy the forearm, while their tendinous
insertions are attached to the phalanges. This explains the fact that
these tendons, seen at the lower part of the forearm, are not visible
superficially during their passage into the palm of the hand.
_The Wrist-Joint._--The _radio-carpal_ joint is formed by the convex
upper surface of the carpus, constituted by the scaphoid, semi-lunar,
and cuneiform bones, articulating with the lower end of the radius and
the fibro-cartilage of the wrist (which lies below the ulna). This
articulation permits movements of the hand in four directions: forwards
and backwards (flexion and extension); outwards and inwards (abduction
and adduction).
_Inter-carpal Joint._--The several carpal bones glide upon one another:
and there is only a limited movement possible of flexion and extension
between the three named bones of the first row (scaphoid, semi-lunar,
and cuneiform) and the four bones of the second row; but lateral
movements are very limited and practically absent.
It is thus obvious that the movements of flexion and extension of the
hand at the wrist-joint are extensive, and amount almost to a right
angle, both before and behind, the mobility of the radio-carpal and
inter-carpal articulations aiding each other in these movements; on
the contrary, the lateral movements of the wrist are more limited, as
they are confined to the radio-carpal articulation, and are restricted
on the outer side (abduction) by the downward projection of the lower
end of the radius. Adduction is a much more powerful movement, rendered
freer by the presence of the triangular fibro-cartilage of the wrist,
and the separation of the ulna and the cuneiform bones. It should also
be noted that in flexion of the hand, when it forms a right angle with
the forearm, the posterior surface of the wrist does not present an
abrupt curve, but rather a rounded form; the convexity being made up
of two series of articulations, the radio-carpal and the inter-carpal
articulations.
[Illustration: FIG. 29.
BONY STRUCTURE OF HAND (anterior or palm surface).--1, 2, 3,
and 4, the four bones of superior row of carpus:--5, 7, 8, and
9, the four bones of inferior row;--10, 10, the five metacarpal
bones;--11, 11, the first phalanges;--12, 12, the second
phalanges;--13, 13, the third phalanges;--14 and 15, the two
phalanges of the thumb.]
The _metacarpus_ (Fig. 29), or skeleton of the palm of the hand,
is composed of five slender, long bones--the five metacarpal
bones--separated from each other by _interosseous_ spaces. Each
metacarpal bone, like the other long bones, is composed of a shaft and
two extremities. The shaft is more or less prismatic and triangular;
the upper or carpal extremity is cuboid, or wedge-shaped; the lower or
digital end is rounded to articulate with the first bone of the finger.
The five bones are distinguished by the names, first, second, third,
fourth, and fifth metacarpal, counting from the thumb to the little
finger; or, again, by the name of the finger to which they correspond
(as the metacarpal bone of the thumb, index finger, etc.). The
first metacarpal bone, or that of the thumb, is the shortest, and is
remarkable for characteristics to which reference will be made later;
the second, or metacarpal bone of the index finger, and the third, or
that of the middle finger, are the longest. The third is generally
longer than the second, so that a line passing through the heads of
the series of metacarpal bones describes a curve with its convexity
downwards, of which the most prominent part corresponds to the head
of the third metacarpal bone. When the hand is firmly closed, and the
fingers bent in the palm, it is the head of the third metacarpal bone
which forms the most prominent part of the fist.
The metacarpal bones articulate with the carpus by their upper
extremities, or bases. In these articulations a very different
arrangement is found for the first metacarpal bone when compared with
that of the other four.
The articulation of the metacarpal bone of the thumb is formed by a
saddle-shaped facet on the trapezium, concave from side to side, and
convex from before backwards, and a corresponding facet at the base
of the first metacarpal bone. It results, then, that as the rider
can move himself on his saddle forwards and backwards, and to either
side, so the metacarpal bone of the thumb is equally movable in all
directions, and can accomplish the movement of circumduction, by
which the extremity of the thumb describes a circle. This mobility
permits the thumb to be separated from the other fingers, or to be
drawn across the hand, or to touch the tips of the other fingers.
This last is called the _movement of opposition_ of the thumb,
and it is owing to this property that the thumb possesses of opposing
itself to the fingers that the hand of man forms such a wonderful
organ for prehension and for performing the most delicate and refined
movements. The articulation of the trapezium and metacarpal bone, which
is the source of these movements, thus deserves particular mention.
The articular surfaces of the two bones are attached to each other by
an articular capsule sufficiently loose to allow all the movements of
which the first metacarpal bone is capable.
On the other hand, the articulations of the metacarpal bones of the
four other fingers do not present any such mobility. In fact, whilst
the base of the first metacarpal bone is free, without being connected
with that of the second, the bases of the other metacarpal bones are
in contact with each other by their lateral surfaces, and are united
by dorsal, palmar, and interosseous ligaments. Again, the transverse
line of union between the second row of the carpus and the base of
these metacarpal bones (carpo-metacarpal line) is irregular, the carpus
and metacarpus being dovetailed into each other, especially at the
level of the second and third metacarpals, by reason of the projection
upwards of the second metacarpal bone, and the projection downwards of
the os magnum (Fig. 29). The carpus and the four last metacarpal bones
therefore form a series of joints, of which the parts are only slightly
movable one on the other, giving a certain elasticity to the whole.
The effects of pressure or sudden shock are avoided by the presence of
numerous parts united in such a manner as to glide one on the other, at
the same time not presenting any independent mobility.
The fingers are formed of a series of slender bones placed end to end,
and termed _phalanges_. Each finger has three phalanges, except the
thumb, which has only two. We distinguish the rows of phalanges by the
names of the first, second, or third, counting from the base to the
free extremity of the fingers; and we give the name of _ungual_ phalanx
to the last because it supports the nail. These phalanges, like the
other long bones, are made up of a shaft and two extremities. The shaft
is semi-cylindrical in shape, rounded behind and flattened in front,
where the flexor tendons of the fingers are lodged. The extremities
present characters which will be pointed out when the articulations of
the fingers are studied.
The articulations of each finger are: the metacarpo-phalangeal
articulation, the articulation of the first with the second, and the
articulation of the second with the third phalanges (inter-phalangeal
articulations).
Each _metacarpo-phalangeal articulation_ is formed by the globular
head of the metacarpal bone being received into a glenoid cavity in the
base of the first phalanx. Such an adaptation of articular surfaces
will permit every kind of movement, and it is easy to understand
that each finger can be bent on the metacarpus, straightened, and
also inclined to either side (abduction and adduction--the act of
separating and bringing together the fingers); but the articular
capsule or fibrous band which surrounds each metacarpo-phalangeal
joint fixes an exact limit to these movements. Flexion is a much
more powerful movement than extension, because the capsules of the
joints are deficient behind, their places being taken by a membranous
expansion of the extensor tendons which passes over the backs of the
joints. Thus extension cannot usually be prolonged further than that
position in which the axis of the fingers forms a straight line with
that of the metacarpal bones, for just then the anterior portion of the
capsule is put on the stretch, and as this part is fibrous, thick, and
resisting, it prevents any increase of extension. When the anterior
ligament is thinner and more relaxed, as sometimes in the female hand,
the fingers can be straightened beyond the straight line, and form
an obtuse angle with the metacarpus. On the other hand, this capsule
is strengthened on either side by a lateral ligament, which, being
inserted at the posterior part of the head of the metacarpal bone, is
put on the stretch when the act of flexion is produced, and when this
act of flexion arrives at a right angle, the lateral ligaments do not
permit it to be carried any farther. It is easy to prove this upon
ourselves, as we cannot flex the first phalanx on the metacarpus beyond
this point, and we cannot, in any case, bring the anterior surface of
the first phalanx of a finger in contact with the palm of the hand, but
only the second and third phalanges.
_Inter-phalangeal Articulations._--The articulations of the
phalanges--that is, those of the first with the second, and those of
the second with the third--are constructed on a different plan from
the metacarpo-phalangeal articulations. Instead of a head received
into a glenoid cavity, we find here, at the inferior extremity of
the first and second phalanges, a surface formed like a pulley, or
trochlea, with two lateral lips separated by a groove or hollow
(Fig. 29); and, on the other hand, on the superior extremity of the
second and third we find two cavities corresponding to the lips of
the pulley, separated by a median projection which corresponds to the
groove. Therefore, given a single phalanx, it will be easy to say
whether it is a first, second, or third phalanx, as the first phalanx
has at its upper end a single articular cavity, while the second and
third have two placed side by side; and again, the third, or ungual,
phalanx may be distinguished at the first glance from the second by the
shape of its free extremity, which is expanded in front into a rough
surface shaped like an arrow-head for the support, not of the nail,
but of the pulp of the finger. The inter-phalangeal joints reproduce
on a smaller scale the pulley, or trochlea, and joint of the elbow,
and present an analogous mechanism permitting only the movements of
flexion and extension. In fact, as each of us may prove upon his
own hand, while the fingers may be moved from side to side at their
metacarpo-phalangeal articulations, the several phalanges can only be
flexed and extended at the inter-phalangeal joints; in other words,
while the finger enjoys great freedom of movement at its base, it only
possesses that of flexion and extension in its component parts. Here
again, and for the same reason, flexion is the more powerful movement.
The movement of extension of the phalanges is limited, because the
anterior portion of the articular capsule put on the stretch by the
movement is stout and strong, but we find a great variety in different
subjects, and with some, such mobility that the terminal phalanges can
be bent backwards. Flexion is limited only by the contact of the soft
parts on the anterior surface of a phalanx.
_The Proportions of the Upper Limb._--Having examined the skeleton
of the upper limb in relation to form and movement, it is necessary
next to study its proportions--namely, to inquire, on the one hand,
what comparison the length of the limb bears to the height, and, on the
other, to compare the length of the different segments of the limb with
each other.
The comparison between the upper limbs and the height may be expressed
in two ways: first, by examining the two arms outstretched in the
horizontal position; the distance which then separates the extremity
of one hand from that of the other is termed the _span of the upper
limbs_, and this transverse measure includes not only the length of
the arms, but also the breadth of the shoulders; secondly, by examining
the upper limb hanging vertically beside the body, and noting to what
level on the lower limb the extremity of the hand (nail of the middle
finger) reaches.
The relation of the span of the upper limbs to the height has been
expressed long since by the formula known as the _square figure of the
ancients_ (Fig. 30). If we draw two horizontal lines, one at the soles
of the feet (_c_, _d_), the other at the summit of the head (_a_, _b_),
and two vertical lines at right angles to the extremities of the two
arms horizontally outstretched, these four lines form by their junction
a perfect square; in other words, the man having the arms horizontal is
enclosed within a square. This shows that the span of the arms is equal
to the height.
[Illustration: FIG. 30.
Figure in a square, ancient method.]
This statement is correct for a man of the Caucasian race of the middle
height, but it is not so for the yellow and black races, in whom the
span of the arm is greater than the height. If from man we pass on to
the anthropoid apes (chimpanzee, gorilla, &c.), we find that the span
of the arms in these becomes more and more extended as compared with
the height. Thus, in the gorilla, the height being 5 ft. 7¼ in., the
span becomes 8 ft. 9¼ in.; and in the chimpanzee, to a height of 5 ft.
5¼ in. the corresponding span is 6 ft. 6 in.
Again, when we examine the upper limbs hanging freely beside the body,
we find that in the European of average height the extremity of the
middle finger corresponds in general to the middle of the thigh; in
subjects of short stature this extremity of the hand descends a little
lower than the middle, and, on the other hand, in very tall men it
ends at a higher level. In the yellow and black races the extremity
of the hand descends much lower than the middle of the thigh; and in
the anthropoid apes we find that, in the chimpanzee, the extremity of
the hand descends below the knee; in the gorilla it corresponds to
the middle of the leg; and, finally, in the orang-outang, and in the
gibbon, it reaches almost to the ankle.
If we seek among the various portions of the upper limb a part which
would answer as a common measure between them, we cannot find anything
satisfactory in this respect. The length of the hand, which would
naturally seem to be indicated as a measure, is not contained an even
number of times in the length of the bones of the shoulder, arm, or
forearm. If, however, we take from the hand the length of the third
phalanx of the middle finger, we have a measure equal to that of the
vertebral border of the shoulder-blade, or of the clavicle. Under those
conditions we may say that the length of the humerus is equal to twice
that of the hand, and that of the forearm equal to one and a half the
length of the hand; but these proportions are so variable that they
cannot be insisted on. We should attach more importance to the rule
that takes the hand as a common measure of the entire body in regard
to height, taking the height as being equal to ten hands. This is a
proportion which often answers in reality, but which presents too many
exceptions to be laid down as a law.
We may here state the fact once for all, that there is not an
_absolute rule_ for the anatomist, or system of proportions
applicable to every class of subject, to those of small as well as
those of large stature. If, on the contrary, an ideal proportion is
adopted, in which a human figure has been altered, so as to correspond
to the abstract conception of beauty, we say that this question of
proportion belongs no longer to the domain of anatomy or observation,
but that here we rather touch æsthetic doctrines; it is for this reason
that we have limited ourselves, when putting forward various ideas of
proportion, to indicating, within such limits as direct observation
permits, whether a part of a limb might serve as a common measure for
this limb or for the total length of the body.
[Illustration: FIG. 31.
The Egyptian Canon.]
The _Egyptian canon_ as demonstrated by Charles Blanc, which has a
certain historical interest, is that the _length of the middle finger_,
taken as a common measure, should be contained nineteen times in the
length of the body. In fact, the “Selection of Funeral Monuments” by
Lepsius (Leipzig, 1852) contains the drawing of a very curious Egyptian
figure, divided by transverse lines into nineteen parts (not including
the head-dress). Now as several passages in different ancient authors
seem to indicate that the Egyptian sculptors have taken the finger as
the unit of the system, Charles Blanc very ingeniously remarks this
fact, that in the figure in question, one of the horizontal lines, the
eighth beginning at the soles of the feet, passes exactly at the base
of the middle finger in the right hand (closed holding a key), while
the seventh touches the extremity of the middle finger of the extended
left hand. It seems to him, then, very probable that the distribution
of these horizontal lines indicates a system of measuring the figure,
and that the space between the seventh and the eighth line measures the
length of the middle finger, which thus becomes the standard of this
system of proportion. According to the Egyptian rule, the length of
the middle finger will be found nineteen times in that of the height
(Fig. 31); it may be that this rule was adopted by the Greek artists,
and Charles Blanc does not hesitate to think that Polycletus, who has
composed a _Treatise on Proportions_, with a model in marble known
by the name of _Doryphorus_, used no other system but the Egyptian;
there has been always found in a number of antique figures this same
proportion of nineteen times the middle finger to the height of the
body, and in the Achilles, for example, the total height does not
exceed by more than ¹⁄₂₀th of an inch the length of the middle finger
multiplied by nineteen.
_Brachial Index._--An interesting proportion to note is that between
the arm and forearm, especially as it has been with anthropologists
the subject of important researches, and will familiarise us with
the term _index_, which we must frequently make use of, especially
when comparing the transverse and antero-posterior diameters of the
cranium. We give, in anthropology, the name _index_ to the number
which expresses the proportion of one dimension to some other, this
last being represented by 100. Supposing, in fact, that we compare one
length, A, equal to one metre, with another length, B, equal to two
metres, in this case, the first length being half that of the second,
we speak of the index found as 50 (because 50 is the half of 100, and
we suppose the second length to be equal to 100). Now the forearm is
shorter than the arm; it represents about three-fourths of it; if,
then, we take the number 100 to represent the length of the humerus,
the number 75, which is three-fourths of 100, would represent the
length of the forearm; and then in denoting by the _brachial index_
the proportion of the length (always shorter) of the forearm with that
of the arm (always longer) we simply say that the brachial index is
represented by 75.
This method of notation, which reduces any numerical proportion to the
decimal system, is very valuable, as it permits us to follow without
difficulty the degree in which a proportion varies according to the
race or species.
Thus we come to speak of the brachial index (proportion of the forearm
to the arm) as 75. We have chosen this particular number in order to
make the example easy; in reality, in adult European subjects this
index is only 74--that is to say, that the forearm is to the arm as 74
is to 100. If we measure the same parts in the adult negro, and reduce
to the decimal proportion the numbers obtained, we find the brachial
index here is 79--or that the forearm is to the arm as 79 to 100. In
the negro, then, the forearm is longer compared with the arm, as 79
is a greater part of 100 than 74. Finally, if we pass on from the
human species to the anthropoid apes, we see that the brachial index
comes to be 80, and even 100--that is to say, that the length of the
forearm becomes equal to that of the arm; and we know, therefore, that
the great length of the upper limbs in the anthropoids (page 86) is
principally owing to the greater length of the forearm. But the most
interesting fact is that in the human race the brachial index is not
the same at different ages--thus, in the European infant at birth this
index is 80; before the end of the first year it is only 77, and by
degrees during childhood it descends until it arrives at 74 in the
adult. This clearly shows that the humerus, during the growth of the
body, lengthens in proportion more than the bones of the forearm;
so that they, which were at first to the humerus as 80 is to 100,
come gradually to be as 77 to 100, and finally as 75 or 74 to 100.
If we were to glance at comparative osteology we should see that, in
such animals as the lion or the horse, the forearm becomes longer in
proportion to the humerus, so as to equal, and afterwards to surpass,
the length of that bone.