=Plantigrades=: =Bear= (Fig. 33).--The scapula of the bear approaches in shape to a trapezium, of which the angles have been rounded off. The anterior border (cervical) is strongly convex in the part next the glenoid cavity. The junction of the superior (spinal) and the cervical border forms almost a right angle, the summit of which corresponds to the origin of the spine. At its posterior angle there is a prominence, directed downwards, the surface of which is hollowed and is separated from the infraspinous fossa by a crest, so that at this level a third fossa is added to the infraspinous one. The neck of the scapula is but slightly marked. The acromion is prominent, and projects a little beyond the glenoid cavity.
The clavicle is rudimentary, but, as an example of the complete development of this bone in plantigrade quadrupeds, we may cite the marmoset.
The humerus is furnished at its superior extremity with a large tuberosity, wide, and situated in front of the head of the bone; the effect of this is that the bicipital groove is internal. As in man, the great tuberosity does not reach so high as the humeral head, but it approaches more nearly to that level. The deltoid impression is very extensive, and descends pretty far down on the body of the bone. The epitrochlea is prominent; the epicondyle is surmounted by a well-marked crest, curved and flexuous in outline.
[Ill.u.s.tration: FIG. 33.--SKELETON OF THE BEAR: LEFT LATERAL SURFACE.
1. Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, fourteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, sternum; 12, ninth and last sternal rib; 13, costal cartilages; 14, acromion process; 15, third fossa on the external surface of the scapula; 16, great tuberosity of the humerus; 17, musculo-spiral groove; 18, epicondyle; 19, radius; 20, ulna; 21, olecranon process; 22, carpus; 23, pisiform; 24, metacarpus; 25, phalanges; 26, ilium, external fossa; 27, pubis; 28, tuberosity of the ischium; 29, obturator foramen; 30, great trochanter of the femur; 31, condyles of the femur; 32, patella, or knee-cap; 33, anterior tuberosity of the tibia; 34, fibula; 35, tarsus; 36, calcaneum, or heel-bone; 37, metatarsus; 38, phalanges.]
The articular surface, which is in contact with the radius, is not a regularly formed condyle; it is a little flattened on its anterior surface, and presents at this level a slight depression which corresponds to a small eminence on the anterior aspect of the superior extremity of the radius. The surface which articulates with the ulna, viewed on its anterior aspect, has the shape of a slightly-marked trochlea; except at the level of the internal lip, which, as in man, descends lower than the surface for articulation with the radius (condyle). Behind, the trochlea is more clearly defined.
The bear possesses a considerable power of rotation of the radius; the bones of the forearm are joined only at their extremities, while in the remainder of their extent they are widely separated. The ulna terminates below in a head and a styloid process; these articulate with the two last bones of the first row of the carpus--viz., the cuneiform and pisiform. The bones of the carpus are seven in number, the scaphoid and the semilunar being fused together.
The metacarpals, five in number, differ very little from one another in regard to length, though they increase in size from the first to the fifth; this may be demonstrated by looking at the palmar surface of the hand. It is the reverse of that which we find in man, for the fifth metacarpal is the thickest of all, and the first is the most slender.
At the level of each metacarpo-phalangeal articulation are two sesamoid bones.
The third digit is the longest. The terminal phalanges present two very different portions: one, the anterior, is curved and pointed; it serves to support the nail, whose shape it a.s.sumes; the other, posterior, forms a sort of sheath into which the base of the nail is received.
The inferior portion of the posterior surface of this latter part articulates with the second phalanx in the case of each of the last four digits, but with the first phalanx in the case of the thumb.
[Ill.u.s.tration: FIG. 34.--SKELETON OF THE DOG: LEFT LATERAL SURFACE.
1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, thirteenth and last dorsal vertebra; 7, lumbar vertebrae; 8, sacrum; 9, coccygeal vertebrae; 10, anterior extremity of the sternum; 11, xiphoid appendix; 12, ninth and last sternal rib; 13, costal cartilages; 14, spinal border of the scapula; 15, supraspinous fossa of the scapula; 16, infraspinous fossa of the scapula; 17, great tuberosity of the humerus; 18, deltoid impression; 19, musculo-spiral groove; 20, olecranon process; 21, radius; 22, carpus; 23, pisiform; 24, metacarpus; 25, sesamoid bones; 26, phalanges; 27, ilium, iliac crest; 28, pubis; 29, tuberosity of the ischium; 30, great trochanter of the femur; 31, patella, or knee-cap; 32, anterior tuberosity of the tibia; 33, fibula; 34, tarsus; 35, calcaneum, or heel-bone; 36, metatarsus; 37, sesamoid bones; 38, phalanges.]
[Ill.u.s.tration: FIG. 35.--SCAPULA OF THE DOG: LEFT SIDE, EXTERNAL SURFACE.
1, Posterior or axillary border; 2, superior or spinal border; 3, anterior or cervical border; 4, spine of scapula; 5, coracoid process; AA", length of spinal border.]
[Ill.u.s.tration: FIG. 36.--LEFT SCAPULA OF THE CAT: EXTERNAL SURFACE.
1, Posterior or axillary border; 2, superior or spinal border; 3, anterior or cervical border; 4, spine of the scapula; 5, coracoid process; AA", length of the spinal border.]
=Digitigrades=: =Cat=, =Dog= (Fig. 34).--In these animals the anterior (cervical) border of the scapula is convex; the posterior (axillary) border is straight or slightly concave. The supraspinous and infraspinous fossae are of equal extent (Figs. 35 and 36). The neck is short. The spine of the scapula becomes more and more prominent towards its inferior extremity, where it ends in a twisted and inflexed portion, which represents the acromion process; this process terminates at the level of the glenoid cavity. The coracoid process is represented by a small tubercle, slightly curved inwards; this tubercle is situated above the glenoid cavity, at the inferior part of the cervical border.
In the dog, the posterior angle, formed by the junction of the axillary and the superior (spinal) borders, is obtuse; the spine rises perpendicularly from the surface of the bone. The width of the scapula, measured at the level of the spinal border (from A to A", Fig. 35), equals about half the length of the spine. We must, however, make an exception for the turnspit dog, in which the superior border equals three-fourths of that length. The scapula is, in this case, of a more compact type; it is broader, but shorter. In the cat, the anterior outline of the scapula, formed by the union of the cervical border and the corresponding half of the spinal, is more convex; the posterior angle is not obtuse, as in the dog. The spine is bent slightly downwards and backwards; before terminating in the acromion process it presents a triangular projection, the apex of which is directed downwards. The tubercle which represents the coracoid process is curved inwards more strongly than that of the dog, thus resembling more closely the appearance of this process in the human being.
All proportions considered, the scapula of the cat is broader than that of the dog; its width, measured along the length of its spinal border (from A to A", Fig. 36), equals three-fourths of the length of the spine.
The clavicle is rudimentary; it is, however, better developed in the cat than in the dog. The clavicle of the cat is represented by a small, elongated bone, curved in outline, the convexity being turned forward; it is united to the acromion and the sternum by ligamentous fibres; that of the dog is merely a scale-like osseous plate situated on the posterior surface of a muscle of this region (see Figs. 16 and 17).
The humerus is long and twisted in the shape of an S. The inferior articular surface has the form of a simple pulley, for the condyle is very slightly marked. The internal part of this articular surface descends lower than the external; this condition resembles that found in the human being, where the inner lip of the trochlea is lower than the condyle.
In the dog, the olecranon fossa communicates with the coronoid by an opening.
In the cat, there is a supra-epitrochlear ca.n.a.l (see Fig. 19), but no olecranon perforation.
The bones of the forearm articulate at their extremities. The body of the radius is united to the body of the ulna by a short, thick, interosseous ligament; the fibres of this ligament, though short, do not prevent the production of some movements at the articulations of the bones.
The radius so crosses the ulna that above, it is in front and external to the latter, while below, it is internal. This bone is flattened from front to back, and slightly convex anteriorly. Its superior extremity is formed, externally, of a portion which represents the head of the radius in man; internally, by another portion which represents half of the coronoid process of the ulna, which, in the human being, belongs exclusively to the latter (see p. 39, the encroachment of the radius on the ulna). This extremity is surrounded with a vertical articular surface which is placed in contact with a small cavity which is hollowed out on the ulna (the lesser sigmoid cavity); and presents at its superior aspect a surface which articulates with the inferior extremity of the humerus. The shaft of the bone has on its internal border rugosities a.n.a.logous to the imprint of the p.r.o.nator radii teres of the human skeleton; these rugosities, indeed, give insertion to a muscle of the same function, and bearing the same name. The inferior extremity, broader than the superior, is hollowed on its external aspect by a small cavity which receives the inferior extremity of the ulna; its inferior surface (concave) articulates with the carpus; its anterior surface (the h.o.m.ologue of the posterior surface of the corresponding extremity of the human radius) presents grooves which serve for the pa.s.sage of the tendons of the muscles which pa.s.s from the forearm to the back of the hand. (For the names of the muscles whose tendons pa.s.s in these grooves, see Fig. 29.)
The ulna is furnished at its superior extremity with an olecranon process, which is more prominent than that of the human ulna; this process is compressed laterally, and its internal surface is hollowed; there we also find a great sigmoid cavity, and a coronoid process situated at the internal part of the anterior surface, a process which, as we have previously shown, it shares with the radius.
The shaft of the bone, prismatic and triangular, diminishes in thickness as it approaches the lower extremity, which articulates with the corresponding extremity of the radius. In the dog, the ulna terminates inferiorly in a blunt point, without enlargement, a.n.a.logous to the head of the human ulna; in the cat, by a head which is prolonged into a styloid process, by which it articulates with a portion of the carpus.
The carpus consists of seven bones--three in the superior row and four in the inferior. In the superior row the scaphoid and semilunar bones are fused together. The pisiform is elongated and expanded at its two extremities; it forms a prominence which, directed backwards, projects beyond the level of the other bones of this region.
The metacarpal bones are five in number; they are enumerated from within outwards; they articulate with the carpus and with each other. The inferior extremity of each metacarpal bone presents the form of a condyle in front; and is divided behind so as to form two lateral condyles, which are separated by a median crest; on these posterior condyles are applied two small sesamoid bones. The metacarpal bone of the thumb is very short; the third and fourth are the longest. The metacarpus, as a whole, is directed vertically.
The phalanges are three in number for each finger, except the thumb, which has but two. The first phalanx, directed almost horizontally forwards, is the longest; the second is directed downwards and forwards; the third consists of two portions: a posterior part, which forms a sort of sheath into which the base of the nail is received; and an anterior, conical in form, and curved in crochet shape, which forms a support for the nail (Fig. 37).
The third and fourth digits are the longest; the second and fifth are of equal length; the thumb is the shortest; it does not touch the ground, and does not even reach the articulation of the metacarpal bone and first phalanx of the second finger.
In the cat, the metacarpal bone of the thumb, although shorter than any of the others, is quite as thick. The third digit is a little longer than either the second or fourth. In animals of this genus, the claws, in the condition of repose, are retracted, and removed from the ground; this prevents their being worn, and thus preserves their sharpness. At such times the third phalanx is received into a groove which is found on the external surface of the second phalanx. In the dog, the claws are not tractile.
[Ill.u.s.tration: FIG. 37.--SKELETON OF THE FINGER OF A FELIDE (LION): LEFT SIDE, INTERNAL SURFACE.
1, Metacarpus; 2, sesamoid bones; 3, first phalanx; 4, second phalanx; 5, third phalanx; 6, gutter for the reception of the base of the nail; 7, prominent osseous crest formed to lodge in the concavity of the nail.]
[Ill.u.s.tration: FIG. 38.--SKELETON OF THE PIG: LEFT LATERAL SURFACE.
1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, fourteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, anterior extremity of the sternum; 12, xiphoid appendix; 13, seventh and last sternal rib; 14, costal cartilage; 15, cartilage of prolongation of the scapula; 16, great tuberosity of the humerus; 17, olecranon process; 18, radius; 19, ulna; 20, pisiform; 21, metacarpus; 22, phalanges of the two great toes; 23, phalanges of the external toe; 24, ilium; 25, pubis; 26, tuberosity of the ischium; 27, great trochanter; 28, knee-cap; 29, anterior tuberosity of the tibia; 30, fibula; 31, tarsus; 32, calcaneum; 33, metatarsus; 34, phalanges of the two great toes; 35, phalanges of the external toe.]
=Unguligrades=: =Pig= (Fig. 38).--The scapula is markedly narrowed in the region above the glenoid cavity. The spine is atrophied at both its extremities, so that at its inferior part we do not find the acromion process. In its middle portion the spine is prominent, and presents a triangular process which turns backwards, overlapping a part of the infraspinous fossa; this latter is much larger than the supraspinous.
The spinal border is surmounted by the cartilage of prolongation, the superior margin of which is convex; this cartilage extends posteriorly beyond the posterior (axillary) border of the bone.
The small tuberosity of the superior extremity of the humerus is but slightly developed; the great tuberosity, on the contrary, is very large. The bicipital groove is situated internal to this. The deltoid impression is scarcely marked.
The forearm is short, directed obliquely downwards and inwards, thus forming with the hand an angle, of which the apex is directed inwards.
The two bones of the forearm are strongly bound to one another by an interosseous ligament, which is formed of very short fibres. The radius appropriates, at its superior extremity, the coronoid process of the ulna. The latter is, notwithstanding, well developed in the rest of its extent; it has a flattened shaft which almost completely overlaps the posterior surface of the radius; its inferior extremity reaches to the carpus.
The carpus is formed of eight bones--four in the superior row, and four in the inferior. The third bone of the superior row (cuneiform) is more in contact with the ulna than with the radius.
There are but four metacarpal bones; there is no metacarpal of the thumb. The two median metacarpal bones are the longest; they are those which correspond to the digits which alone touch the ground. The internal digit and the external one are thin and short; they are functionless, as a rule, taking no part in supporting the limbs on the ground. Notwithstanding this, they are formed, as the other digits, of a number of phalanges, which give them the semblance of perfect digits.
(We shall soon see that in certain animals there exist digits which, being incomplete with regard to the numbers of their const.i.tuent bones, more accurately merit the name of imperfect digits.)
The third phalanges are each enclosed in a h.o.r.n.y hoof, to which the name of _onglon_ has been given.
We have already drawn attention to the smaller lateral digits, and noted the general fact that they do not come in contact with the ground.
It is necessary to modify this statement by adding that under certain conditions they give a slight amount of support; for example, when the individual is the subject of excessive obesity, the limbs yield under the weight, and the nails of the lateral digits may touch the ground.
A similar fact may be noticed in pigs of ordinary bulk at the moment when, during walking, each of the fore-limbs commences to bear the weight--that is to say, when it is directed obliquely downwards and forwards; then all the digits are in contact with the ground.
=Unguligrades (Ungulates)=: =Sheep=, =Ox= (Fig. 39).--The scapula, which is of elongated form, is very narrow in the vicinity of the glenoid cavity. The spine, which becomes more and more salient towards its inferior part, terminates abruptly in a border, which, forming an acute angle with the crest, produces a projection which represents the acromion process--a very rudimentary acromion, for it does not reach the level of the glenoid cavity. The supraspinous fossa is much smaller than the infraspinous; it hardly equals one-third the extent of the latter.
The anterior border, thin and convex in its superior portion, is concave in the rest of its extent; the posterior border is thick and slightly concave; the spinal border is surmounted by the cartilage of prolongation. In the ox the spine of the scapula, in its middle portion, is flexed a little backwards on the infraspinous fossa.
[Ill.u.s.tration: FIG. 39.--SKELETON OF THE OX: LEFT LATERAL SURFACE.
1, Cranium; 2, face; 3, atlas; 4, axis; 5, seventh cervical vertebra; 6, first dorsal vertebra; 7, thirteenth and last dorsal vertebra; 8, lumbar vertebrae; 9, sacrum; 10, coccygeal vertebrae; 11, sternum; 12, xiphoid appendix; 13, eighth and last sternal rib; 14, costal cartilages; 15, spine of scapula; 16, cartilage of prolongation of the scapula; 17, great tuberosity of the humerus; 18, musculo-spiral groove; 19, olecranon process; 20, radius; 21, carpus; 22, pisiform; 23, metacarpus; 24, rudimentary metacarpal; 25, sesamoid bones; 26, first phalanges; 27, second phalanges; 28, third phalanges; 29, anterior iliac spine; 30, pubis; 31, tuberosity of the ischium; 32, great trochanter; 33, supracondyloid fossa of the femur; 34, patella, or knee-cap; 35, anterior tuberosity of the tibia; 36, fibula; 37, coronoid tarsal bone; 38, tarsus; 39, calcaneum; 40, metatarsus; 41, rudimentary metatarsus; 42, sesamoid bones; 43, first phalanges; 44, second phalanges; 45, third phalanges.]