Descriptive Anatomy in Birds

Birds Anatomy - Ontogeny and Phylogeny

Descriptive Anatomy in Birds

Descriptive Anatomy requires a number of technical terms which shall not be ambiguous, or permit of doubt as to their intended meaning. For instance, terms like upper and lower, anterior and posterior, inner and outer, are often liable to be misunderstood. In ordinary parlance anterior corresponds with ventral in Man (with reference to whom many of our technical terms have been invented), but the head though at the anterior end of the animal is not ventral, and yet the anterior surface of a vertebra may mean its ventral surface. In fact, these vernacular names change their meaning according to the starting-point which happens to be used.

It seems therefore advisable to enumerate, and give a definition of, those terms which it is useful to apply throughout in the description of the various organs of a Bird.

The longitudinal axis of every bird corresponds with its vertebral column: one end is marked by the head, the other by the tail, thus giving the terms cephalic and caudal; and concerning the neck, trunk, and tail, together with their constituent parts, anterior and posterior. On one side of the vertebral column or axis are situated the heart, lungs, and digestive organs; this is the ventral, in opposition to the dorsal side. These give, combined with anterior and posterior, right and left. An axis at right angles with the longitudinal one, and at the same time running right and left, is a transverse axis; beginning with the vertebral axis as the starting-point, the terms proximal and distal are applied to any organ or part which is referable to the longitudinal axis. These two terms are chiefly applicable to parts like ribs and limbs with their various elements. The proximal end of the tibia articulates with the distal end of the femur; the proximal end of a rib articulates with a vertebra, and so on. The tip of the wing marks its distal, the Axilla its proximal end.

With reference to an ideal plane through the longitudinal axis, and at right angles to the transverse axis, are applied the terms median or inner, lateral or outer. Lastly, since it is not always obvious to which axis or plane a given organ is to be referred, its parts can be described with reference to its neighbours. Hence we speak of the tibial and fibular, radial and ulnar side of the bones and other parts of the extremities; the fourth toe is on the fibular, outer, or lateral side of the foot, the first, which is ordinarily the hind toe, on the tibial, inner, and posterior side.

The basal part of an organ is generally also its proximal part or root, while the apex corresponds with its free or distal end, the latter being the portion most removed or distant from the region whence it grew. Thus we speak of the distal tracheal rings as joining the bronchi, while proximally the trachea is attached to the larynx.

Homologous Organs

In comparing the various parts of one animal with each other, or with those of another animal, we call the organs which are morphologically or structurally similar homologous, the parts which physiologically or functionally correspond are analogous. When the comparison is restricted to one individual, the homologies are general. The different vertebrae, or the ribs, or the anterior and posterior extremities of any particular Bird are serially homologous or homodynamous organs, because they are to a certain extent repetitions of each other, although not necessarily exactly alike. If the comparison refers to similar organs in various individuals, no matter if these belong to the same species, genus, family, or class, the homologies are special, and these again may be complete or incomplete. For instance, the humerus of a Bird is completely homologous with that of a Mammal, Reptile, or Amphibian; the atlas or first vertebra of a Crow is completely homologous with the same part of a Dog. On the other hand, the wing of a Orow is only incompletely homologous with the arm of Man; nor is the two-toed foot of the Ostrich completely homologous with the four-toed foot of a Fowl, although the various bones which compose the feet in both are complete homologues.

Homologous organs are consequently developed from the same parts of the embryos of the creatures which are under comparison. Hence the number of existing homologies in given animals indicates their further or closer relationship, and is used for assigning these animals to their places in the system. It follows from this consideration that the animal's place in the system depends greatly, or entirely upon the characters or organs selected for this purpose.  Unless all the organs and all their characters are carefully considered, not only on the few Birds which happen to occupy our attention at the time, but also in Birds of as many different groups as it is possible to examine, our attempts to produce a classification of Birds must invariably end in the production of arbitrary "keys." It is extremely difficult, often hopeless, with the present state of our knowledge of the anatomy of Birds, to decide which characters and which organs are of extrinsic taxonomic value, and which are not. Nor is it always possible to see why certain organs, fully developed, and exhibiting striking and constant features in one group of Birds, are extremely variable in another otherwise very circumscribed and apparently natural group.  Supposing such a character to be absent in a given group, is it absent because it has not yet been developed, or is it because it has been lost?  Has it been lost by the ancestors of this group, or has it been abolished within this group? In the former case the absence of this character would probably help to decide the relative position of the group; in the latter case this very same character would be reduced to a diagnostic point within the group, and not throw any light upon its relationship or systematic position. It may be very easy to diagnose genera or even large groups of birds, but this ability to determine them by the help of mechanically arranged "keys" does not necessarily afford us more than an occasional glimpse of the sunk avine tree, at the reconstruction of which we all aim, as the true representation of the natural affinities of Birds.

It is occasionally insisted upon that "tact" will help us to select and to reject characters, and thus prevent us from falling into glaring errors; but tact is a personal feeling, often bias, and it is proof, not inclination, that settles scientific questions. The importance of these considerations, often expressed before in abler words, is gaining more and more ground among ornithologists, and will therefore permit the following illustrations of the ways in which we may or may not apply the study of comparative anatomy to classification.

Illustration: Presence of the Ambiens and Caeca

The presence of the Ambiens Muscle is a Reptilian feature; among Birds it exists in the majority of the lower groups, and is absent in most of the higher members of the Class. We conclude that the latter have lost this muscle, and not that it has not yet been developed in them. Its reduction or loss is still going on within some groups, such as Parrots and Pigeons. This loss takes place independently in widely different groups. It follows, first, that absence of this muscle does not always indicate relationship; secondly, that we can derive forms that are without it from a group which still possess it; but that the reversed conclusion is not possible. We know of no organ which has been redeveloped after it has once disappeared in the ancestors of the animals under consideration. Therefore the absence of the ambiens muscle in all Owls, which apparently use their hinder extremities in the same way as the Falconidae (which possess this muscle), indicates that the Owls are not developed from the Falconidae, but from a group which, like the Macrochires, had already lost this organ.

Similar arguments apply to the caeca. It is generally admitted that the ancestral bird-stock did possess well-developed caeca, therefore all those birds which are now found without caeca must have lost them either phylogenetically or even during their embryonic development. In fact, we find in embryos of such birds as have, when adult, only very small or rudimentary caeca, that the germs of these organs are, in the embryo, just as well developed as in birds with long caeca; but these organs, in a Pigeon for instance, do not grow any further. They are in early life stopped in their development, and thus remain in a rudimentary state. Again, in all those birds which are completely devoid of caeca, their suppression is simply carried out to the extreme. We cannot therefore, as has been done sometimes, separate Birds into those with and those without caeca: this is especially wrong, as there exist many forms, which, although undoubtedly allied to each other, differ greatly in the presence or absence of these organs. If we want to use the caeca as a differentiating character, we must consider their quality, and enquire whether those organs are functional and well developed, or are they now without function?  Consequently birds with rudimentary caeca have to be grouped together with those which have no caeca, although the ancestors of both had functional caeca; and since we know that these organs stand in close correlation with the nature of the food, we are enabled to weigh their taxonomic value. Hence it is probable that the Owls are related to the caeca-possessing Nightjars, and that the caecaless Macrochires (like Swifts) are a recent offshoot of the latter, while it is impossible to assume that the Owls are descendants of the Diurnal Birds-of-Prey.

The modifications of the Carotid Arteries enabled Professor Fuerbringer to draw a very ingenious and valid conclusion as to the probable original centre of the Parrots. While the Australian, Oriental, and African Parrots exhibit almost every possible modification of these arteries, from the most primitive to the most specialised conditions, the American Parrots possess only the right deep carotis and a left superficial carotis, an arrangement which is a decidedly recent, not primary feature. Hence the conclusion that the American Parrots are a branch of the Palaeotropical stem; but however fascinating such speculations are, we must not forget that they hardly ever amount to definite proofs.

Supposing we divide Birds into two classes (A and B), according to the presence or absence of the Ambiens muscle. As a second differentiating character let us take the functional or fully developed (a) and the absent or functionless state of the CAECA (b); and as a third character the presenc (Ð) or absence (ß) of an Aftershaft.  Then using the ambiens as the principal, and the aftershaft as the tertiary differentiating feature, and indicating presence or absence by the signs + and - respectively, we get the following eight divisions:-

A. Ambiens +
        a. Caeca +
               Ð. Aftershaft + e.g. Gallinae, Impennes, Pheonicopterus, Musophaga, etc.
               ß. Aftershaft - e.g. Anseres, etc.
        b. Caeca -
               Ð. Aftershaft + e.g. Accipitres, Psittaci partim.
               ß. Aftershaft - e.g. Columbae partim.
B. Ambiens -
        a. Caeca +
               Ð. Aftershaft + e.g. Alca, Podicipes.
               ß. Aftershaft - e.g. Striges.
        b. Caeca -
               Ð. Aftershaft + e.g. Psittaci pt., Cypseli, Trochili, etc.
               ß. Aftershaft - e.g. Passeres, Columbae pt., Herodii, etc.
Thus the Owls in this arrangement approach nearest to the Auks and Grebes, while the Parrots, owing to their variable ambiens muscle, are grouped either with the Accipitres, or with the Swifts and Humming-birds. This is obviously unsatisfactory, perhaps owing to the value of the ambiens muscle being overrated. Let us next use the aftershaft as the principal, the ambiens as the secondary determining character, and the caeca as the third. Then the Psittaci approach the Gallinaceous birds and also the Auks and Grebes, while the Owls verge into the neighbourhood of Pigeons, Herons, and Passerine birds. Again, by using the caeca as the principal, and the ambiens as the secondary feature, Psittaci, Accipitres, and Columbre, Owls, Auks, and Grebes are once more thrown together. The same or very similar arrangements result from a combination of the caeca with the oil-gland, or of the ambiens and caeca with the conditions of the palatal bones. But these persistent coincidences will never induce us to look upon them as indicating relationship between Owls, Auks, and Grebes, because this conclusion would be obviously wrong! How does the question stand with regard to other combinations, when we cannot at a glance discern a glaring error?  When, e.g. according to the muscles of the thigh, leaving out the ambiens, Striges, Accipitres, and Cypselidae stand closely together?  Is this a mere coincidence or does a deeper meaning underlie this Trias? It is obviously not due to a superior taxonomic value of Garrod's myological formulae, because application of the same principle throws Nightjars, Storks, and Parrots together.


It is hopeless to attempt to arrive at a natural classification of Birds by a mechanical arrangement of even a great number of alleged leading characters. More may be expected from the combination of various taxonomic arrangements, each of which has been based upon a single organic system without reference to other organs. Of course everyone of such one-sided attempts will occasionally show a rather perplexing face, but each of them will bring to light some unexpected points of resemblance between certain groups; and, while restricting ourselves to one organic system, we are more likely to understand which points are given to modifications through mode of life, food, habit, and surroundings, and which remain least affected, and therefore are indicative of relationship. Let us then combine the several one-sided arrangements. They will each of them contribute something good or certain, and thus help to settle the great question. Reasoning from a broad basis of facts will do the rest.

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