Much labour has been bestowed by A.H. Garrod and Max Fuerbringer upon
the investigation of the variations of the inserting tendons of the
patagial muscles (fig. 14), mainly from a taxonomic point of view. The
_propatagialis longus_ muscle is composed of slips from the deltoid,
pectoral, biceps and _cucullaris_ muscles. Its strong belly originates
near the shoulder joint from clavicle, coracoid and scapula. Its
elastic tendon runs directly to the carpus, forming thereby the outer
margin of the anterior patagium, or fold of skin between the upper and
forearm, which it serves to extend, together with the _propatagialis
brevis_ muscle. This runs down the anterior and outer side of the
upper arm, and is attached to the proximal tendon of the _extensor
metacarpi radialis longus_, a little below the outer condyle of the
humerus. In most birds the tendon is split into several portions, one
of which is often attached to the outer side of the ulna, below the
elbow joint, while others are in variable but characteristic ways
connected with similar slips of the _propatagialis longus_. The
posterior patagium, the fold between trunk and inner surface of the
upper arm, is stretched by the _metapatagialis_ muscle, which is
composed of slips from the _serratus, superficialis, latissimus dorsi_
and the _expansor secundariorum muscles_. This, the stretcher of the
cubital quills, is a very interesting muscle. Arising as a long tendon
from the sterno-scapular ligament, it passes the axilla by means of a
fibrous pulley, accompanies the axillary vessels and nerves along the
humerus, and is inserted by a few fleshy fibres on the base of the
last two or three cubital quills. Here, alone, at the distal portion
of the tendon, occur muscular fibres, but these are unstriped,
belonging to the category of cutaneous muscles. We have here the
interesting fact that a muscle (portion of the _triceps humeri_ of the
reptiles) has been reduced to a tendon, which in a secondary way has
become connected with cutaneous muscles, which, when strongly
developed, represent its belly.
The _flexor digitorum sublimis_ muscle arises fleshy from the long
elastic band which extends from the inner humeral condyle along the
ventral surface of the ulna to the ulnar carpal bone, over which the
tendon runs to insert itself on the radial anterior side of the first
phalanx of the second digit. Owing to the elasticity of the
humerocarpal band the wing remains closed without any special muscular
exertion, while, when the wing is extended, this band assists in
keeping it taut. The arm-muscles have been studied in an absolutely
exhaustive manner by Fuerbringer, who in his monumental work has
tabulated and then scrutinized the chief characters of fourteen
selected muscles. The results are as interesting from a morphological
point of view (showing the subtle and gradual modifications of these
organs in their various adaptations), as they are sparse in taxonomic
value, far less satisfactory than are those of the hind-limb. He was,
however, the first to show clearly that the Ratitae are the retrograde
descendants of flying ancestors, that the various groups of surviving
Ratitae are, as such, a polyphyletic group, and he has gone fully into
the interesting question of the development and subsequent loss of the
power of flight, a loss which has taken place not only in different
orders of birds but also at various geological periods, and is still
taking place. Very important are also the investigations which show
how, for instance in such fundamentally different groups as petrels
and gulls, similar bionomic conditions have produced step by step a
marvellously close convergence, not only in general appearance, but
even in many details of structure.
Of the muscles of the hind-limbs likewise only a few can be mentioned.
The _ambiens_ muscle, long and spindle-shaped, lying immediately
beneath the skin, extending from the pectineal process or ilio-pubic
spine to the knee, is the most median of the muscles of the thigh.
When typically developed its long tendon passes the knee-joint,
turning towards its outer side, and lastly, without being anywhere
attached to the knee, it forms one of the heads of the _flexor
perforatus digit_, ii. or iii. One of the functions of this peculiar
muscle (which is similarly developed in crocodiles, but absent, or not
differentiated from the ilio-tibial and ilio-femoral mass, in other
vertebrates) is that its contraction helps to close the second and
third toes. Too much has been made of this feature since Sir R. Owen
(_Cyclop. Anat. Phys_. i. p. 296, 1835), following G.A. Borelli (_De
motu animalium_, Rome, 1680), explained that birds are enabled to
grasp the twig on which they rest whilst sleeping, without having to
make any muscular exertion, because the weight of the body bends the
knee and ankle-joints, over both of which pass the tendons of this
compound muscle. There are many perching birds, e.g. all the Passeres,
which do not possess this muscle at all, whilst many of those which
have it fully developed, e.g. Anseres, can hardly be said to "perch."
Garrod went so far as to divide all the birds into _Homalogonatae_ and
_Anomalogonatae_, according to the presence or absence of the ambiens
muscle. This resulted in a failure. To appreciate this, it is
sufficient to enumerate the birds without the critical muscle:
_Passeriformes_ and _Coraciiformes_, without exception; _Ardeae_ and
_Podiceps_; lastly various genera of storks, pigeons, parrots, petrels
and auks. The loss has taken place, and still takes place,
independently in widely different groups. It follows, first, that the
absence of this muscle does not always indicate relationship; secondly
that we can derive birds that are without it from a group which still
possess it, but not vice versa. The absence of the ambiens muscle in
all owls, which apparently use their feet in the same way as the
_Accipitres_ (all of which possess it), indicates that owls are not
developed from the latter, but from a group which, like the other
_Coraciiformes_, had already lost their muscle.
Garrod further attributed much taxonomic value to the
_caudilio-femoralis_ muscle (fig. 15). This, when fully developed,
consists of two parts, but inserted by a single ribbon-like tendon
upon the hinder surface of the femur, near the end of its first third;
the caudal part, _femoro-caudalis_, expressed by Garrod by the symbol
_A_, arises from transverse processes of the tail; the iliac part
(_accessoro-femoro-caudal_ of Garrod, with the symbol _B_), arises
mostly from the outer surface of the postacetabular ilium. Of course
this double-headed condition is the more primitive, and as such exists
in most nidifugous birds, but in many of these, as well as in many
nidicolous birds, either the caudal or the iliac head is absent, and
in a very few (_Cancroma, Dicholophus, Steatornis_ and some
_Cathartes_) the whole muscle is absent. The _caud-ilio flexorius_
(_semitendinosus_ of most authors) arises from the transverse
processes of the tail, and from the distal half of the postacetabular
ilium, thence passing as a broad ribbon to the popliteal region, where
it splits into two portions. One of these, broad and fleshy, is
inserted upon the posterior surface of the distal third of the femur.
This portion, morphologically the original, was named the "accessory
semitendinosus" with the symbol _Y_; the other portion descends on the
hinder aspect of the leg and joins the fascia of the inner femoral
head of the _gastrocnemius_ muscle. In many birds the insertion is
shifted from the femur to the neck of the tibia, in which case the
"accessory head" is said to be absent, a condition expressed by Garrod
by the symbol _X_. By combining the four symbols _A, B, X, Y_,
according to their presence or absence, Garrod got a considerable
number of formulae, each of which was overruled, so to speak, by the
two categories of the presence or absence of the _ambiens_ muscle. It
needs hardly to be pointed out why such a purely mechanical scheme was
doomed to failure. Its author, with a considerable mathematical and
mechanical bias, reckoned entirely with the quantity, not with the
quality of his units, and relied almost implicitly upon his formulae.
It is, however, fair to state that his system was not built entirely
upon these muscular variations, but rather upon a more laborious
combination of anatomical characters, which were so selected that they
presumably could not stand in direct correlation with each other,
notably the oil-gland, caeca, carotids, nasal bones and above all, the
muscles of the thigh. He was, indeed, the first to show clearly the
relationship of the heron-like birds with the Steganopodes; of
stork-like birds with the American vultures; the great difference
between the latter and the other birds of prey; the connexion of the
gulls and auks with the plovers, and that of the sand-grouse with the
pigeons--discoveries expressed in the new terms of the orders
Ciconiiformes and Charadriiformes. These are instances, now well
understood, that almost every organic system, even when studied by
itself, may yield valuable indications as to the natural affinities of
the various groups of birds. That Garrod has so very much advanced the
classification of birds is ultimately due to his comprehensive
anatomical knowledge and general insight.
[Illustration: FIG. 15.--Left thigh-muscles of a Rail. Outer view
after removal of the Il.fb, ilio-fibularis and Il.tib, ilio-tibialis.