[15] In spite of the fact that the custom is to designate the terminal portions of the foot of birds by the name of digits, we prefer to employ here the terms _foot_ and _toes_. In adopting this decision we believe we are acting according to a more didactic method. h.o.m.ology of names should, in our opinion, always accompany h.o.m.ology of regions.

[16] With regard to the gallinaceae, we must add that in certain varieties the number of toes is five; those which are directed forwards are three in number; the internal one which pa.s.ses backward, is double. The two toes which are the subject of this special arrangement are placed very close together, and are nearly always superimposed. This condition is found in the Houdan and Dorking breeds.

[Ill.u.s.tration: FIG. 47.--POSTERIOR LIMB OF THE HORSE PLACED IN THE POSITION WHICH IT SHOULD OCCUPY IF THE ANIMAL WERE A PLANTIGRADE: LEFT LIMB, EXTERNAL SURFACE.

1, Tibia; 2, astragalus; 3, calcaneum; 4, metatarsus; 5, first phalanx; 6, second phalanx; 7, third phalanx.]

In climbing birds (parrots, woodp.e.c.k.e.rs, and toucans), the innermost toe is not only directed backward, but the external toe accompanies it in that direction; consequently, there are two posterior and two anterior toes. Sometimes they are all directed forwards; this disposition is found in the martins. In some birds, the number of toes is reduced to three: the ca.s.sowary shows this reduction; in others, the number is still further diminished--the ostrich, for example, has but two.

[Ill.u.s.tration: FIG. 48.--SKELETON OF THE FOOT OF A BIRD (THE c.o.c.k): LEFT SIDE, EXTERNAL SURFACE.

1, Metatarsus; 2, spur; 3, rudimentary metatarsal; 4, first toe; 5, second toe; 6, third toe; 7, fourth toe.]

Further, we find that, in general, the number of the phalanges increases, when we examine the toes in commencing with the most internal (Fig. 48): this has two; then the following one three; that which comes next in order has four; and the most external toe has five. The phalanges of this last are short; so that, although it is formed by a larger number of bones, it is not the longest of the toes.

THE POSTERIOR LIMBS IN SOME ANIMALS.

=Plantigrades=: =Bear= (Fig. 33, p. 50).--The external iliac fossa is very deep. The femur is longer than the bones of the leg; the great trochanter does not reach the level of the head of the femur. The fibula is well developed; it is united to the tibia at its superior and inferior extremities only.

The foot, which, as in the case of the hand, rests on the ground by the whole extent of its plantar surface, presents five toes; the shortest of these is the internal--that is, the toe which corresponds to the great-toe in man; the third and fourth are the longest, and they are almost equal; there is a very slight difference in favour of the fourth, which is slightly superior in dimensions to the third.

=Digitigrades=: =Cat=, =Dog= (Fig. 34, p. 52).--The external iliac fossa, which looks outwards, is deep; the iliac crest is convex anteriorly, the convexity is continued from one iliac spine to the other.

[Ill.u.s.tration: FIG. 49.--PELVIS OF THE DOG, SEEN FROM ABOVE.

1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac diameter; BB', bi-ischial diameter.]

In the dog, the distance which separates the anterior iliac spines is less than that which separates the ischia (Fig. 49). On a skeleton which we measured, the transverse diameter, the distance from the anterior iliac spine of one side to that of the opposite side, was 8 centimetres, whilst the distance which separated the ischia was 105 millimetres; on another skeleton, the first measurement was 127 metres, and the second was 146 millimetres. It seems to us unnecessary to multiply examples.

In the cat, the iliac spines are but slightly marked; the result is that the iliac crest is almost confounded with the inferior and superior borders of the ilium. The two diameters referred to above are almost equal (Fig. 50).

We draw particular attention to what we have just noted in regard to the transverse proportions of the iliac and ischiatic regions of the dog and the cat. These relations are evidently of importance with regard to shape, since the iliac crests and the ischia are noticeable beneath the skin.

[Ill.u.s.tration: FIG. 50.--PELVIS OF A FELIDE (LION), VIEWED FROM ABOVE.

1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac diameter; BB', bi-ischial diameter.]

In the dog, the shaft of the femur is slightly convex in front; but in the cat it is straight. The borders of the shaft are slightly marked, so that it is almost cylindrical. The _linea aspera_, less prominent than in man, gains in width what it loses in elevation; it const.i.tutes what may almost be called a rough _surface_. This surface is narrower in its middle portion than at its extremities, where it bifurcates to go upwards to the two trochanters, and downwards to the two condyles. At the superior extremity, the neck is short, the great trochanter reaching almost to the level of the head of the femur; the digital _cavity_, which is situated on the internal surface of the great trochanter, is very deep. At its inferior extremity it projects strongly backward. The trochlea is narrow; in the cat its two lips are equally prominent, while in the dog the external is a little more elevated than the internal, which on its part is a little thicker. The trochlea is still more independent of the condyles than in the human femur; it is separated from these latter by a slight constriction.

The knee-cap is long and narrow.

The tibia of the dog is slightly curved from before backward: it has the form of an elongated S; this conformation is in great part due to the very marked projection of the anterior tuberosity and of the superior portion of the crest, which, a little below that tuberosity, turns abruptly backwards, and thus describes a curve the concavity of which is directed forward. The superior part of the external surface is very much hollowed out.

The superior extremity is much thicker than the inferior one. It is not only wide in the transverse direction, but is more especially extended from before backwards; the prominence of the anterior tuberosity is the cause of the elongation of this antero-posterior diameter. On the posterior part of the external tuberosity is found a surface to which the superior extremity of the fibula is applied.

The inferior extremity presents an articular surface, which is formed of two lateral cavities, separated by a crest, which is directed obliquely forwards and outwards. The internal part is prominent, and forms the internal malleolus.

With regard to the fibula, it is united to the tibia by its extremities and by the inferior half of its shaft. This latter is more expanded below than in its upper part. The superior extremity is flattened from without inwards. The inferior extremity projects beyond the articular surface of the tibia, and forms the external malleolus, which, instead of, as in man, descending further than the tibial malleolus, stops at the same level, and even descends a little less than does the latter.

In the cat, the curve of the tibia is less p.r.o.nounced; this is due to the fact that the crest, instead of being concave in its middle portion, is slightly convex anteriorly. The fibula, less flattened than that of the dog, is united to the tibia by its extremities only, and is separate in the rest of its extent.

The bones of the tarsus are seven in number, and arranged as in man, with this difference (which is easily comprehended), that their general relations are changed on account of the vertical direction of the tarsus. For example, the astragalus, instead of being above the calcaneum, is situated in front of it; the cuneiform bones, instead of being situated in front of the scaphoid, are found below it, etc.

These animals have but four well-developed metatarsals; that which corresponds to the great-toe is represented merely by a small style-shaped bone, situate at the internal part of the region.

Nevertheless, we find this toe fully developed in some dogs.

Notwithstanding this, the bones which form it are, however, but rudimentary, and much smaller than those of the innermost digit of the fore-limb.

Sometimes it is double; this condition is demonstrable in individuals belonging to breeds of large size. The median metatarsals are more fully developed than the other bones of the same region which are next them.

Viewed as a whole, the metatarsal bones are a little longer than the metacarpals; the result is that the distance which separates the tarsus from the ground is a little greater than that which separates the carpus from the plane on which the anterior limbs rest. The length of the calcaneum still further exaggerates this difference, and, as in the animals with which we shall occupy ourselves later on, the projection which this bone forms is distinctly higher than that which is produced by the pisiform.

The metatarsus, as a whole, is a little narrower than the metacarpus; not only on account of the presence of a thumb in the anterior limb, but, further, because the bones of this latter region are wider than those of the corresponding part of the posterior limb.

The phalanges closely resemble those of the anterior limbs.

=Unguligrades=: =Pig= (Fig. 38, p. 58).--The pelvis in this animal presents a few of the characters which we shall again meet with in the ruminants and the solipeds; however, the posterior (or internal) iliac spines are relatively more widely separated from one another than in the latter. This arrangement reminds us of that found in the carnivora.

The femur presents nothing very special. The knee-cap is thick, and ovoid in outline.

The fibula is completely developed, as in the carnivora; and is connected with the tibia at both its extremities.

The tarsus consists of seven bones. The astragalus and the calcaneum differ slightly from those of ruminants.

The foot, like the hand, has two median digits which rest on the ground by their third phalanges; and an internal and an external digit, which are removed from it. The metatarsals are a little longer than the metacarpals.

[Ill.u.s.tration: FIG. 51.--PELVIS OF THE OX: SUPERIOR SURFACE.

1, Iliac crest; 2, external iliac fossa; 3, sacrum; AA', bi-iliac diameter; BB', bi-ischiadic diameter.]

=Unguligrades=: =Sheep=, =Ox= (Fig. 39, p. 61).--The pelvis of ruminants of this group closely resembles that of the horse, which we will study later on (see p. 99). That which we must at once point out is that, with regard to the ratio formed by a comparison of the bi-iliac and bi-ischiatic diameters, it may be placed between the ratio obtained in comparing those diameters in the pelvis of the carnivora and that of the solipeds. Indeed, in the ruminants, the distance which separates the ischia exceeds the width of one iliac only, and does not equal, as in the felide, the total width of the anterior part of the pelvis (Fig.

51). In the skeleton of the ox, which forms part of the anatomical museum of the ecole des Beaux-Arts, the bi-ischiadic diameter is 39 centimetres, whilst the width of one iliac crest is 29 centimetres, so that, in contrast to that which we find in the dog, the width of the ischiadic region is less than that formed in front by the addition of the iliac crests.

The great trochanter is large, and extends beyond the level of the plane in which the head of the femur is found.

In the ox, the linea aspera, instead of being a narrow crest, is spread out, and forms in reality a surface; the posterior surface of the femur.

At the inferior and external part of this surface is situated a cavity which surmounts the corresponding condyle, and is known as the _supracondyloid fossa_. On the internal part of the same region there are a series of tubercles, which, because of their position in relation to the corresponding condyle, const.i.tute the _supracondyloid crest_.

The internal lip of the trochlea is much thicker and much more prominent than the external.

The details which we have just now examined in connection with the ox are less marked in the sheep.

The trochlea, narrow as a whole, is clearly separate from the condyles by a very marked constriction.

The patella, which is thickened in the antero-posterior direction, has the shape of a triangular pyramid with the base upwards. Its posterior surface, which articulates with the trochlea, presents an arrangement which is adapted to the disposition of this latter--that is to say, the surface which is in contact with the internal lip is larger than that which articulates with the lip of the opposite side.

The tibia of the ox is proportionately shorter than that of the sheep.

The shaft of this bone is flattened from before backwards, in its inferior half. The median crest of the articular surface of the inferior extremity is the most prominent part of that region.