The dorsal vertebrae form the posterior limit in man, and superior in quadrupeds, of the region of the trunk known as the _thorax_. A single bone, the sternum, is situated at the aspect opposite; the ribs bound the thorax on its sides.
In its general outlines the thorax in quadrupeds resembles that of man--that is to say, that, as in the latter, the anterior portion--superior in the human being--is narrower than the part opposite. But the progressive widening takes place in a more regular and continuous fashion, so that it presents a more definitely conical outline. This purely conical form is nevertheless found in the human species, but only during infancy; the inferior portion of the thoracic cage being then widely expanded, because of the development of the abdominal viscera, which at that period are relatively large.
But the proportionate measurements of the thorax are different. Indeed, we may recall that in man the thorax is flattened from before backwards, so that the distance between the sternum and the vertebral column is shorter than the distance from the rib of one side to the corresponding one of the opposite side (Fig. 5). In animals, on the contrary, it is flattened laterally. Its vertical diameter--measured from the sternum to the vertebral column--is greater than the transverse measurement (Fig.
6).
[Ill.u.s.tration: FIG. 5.--A TRANSVERSE SECTION OF THE THORAX OF A MAN PLACED VERTICALLY--THAT IS TO SAY, IN THE DIRECTION WHICH IT WOULD a.s.sUME IN A MAN PLACED IN THE ATt.i.tUDE OF A QUADRUPED (A DIAGRAMMATIC FIGURE).
1, Dorsal vertebra; 2, sternal region; 3, costal region of one side; 3", costal region of the other side.]
From this results a peculiar arrangement of the muscles that we are able to bring directly into prominence, which presents points of interest from the point of view of the contraction of the subcutaneous layer.
Indeed, in man the region occupied by the pectorals is very broad; it is a wide surface turned directly forward. In quadrupeds, this region of the pectorals is narrowed. It is not spread out, as in the preceding instances; and the appearance it presents is explained by the fact that the thorax is compressed laterally. If we examine the thorax on one of its lateral surfaces, the muscles, on the contrary, are more extended.
We see the contour of the vertebral column, and the median part of the abdomen; and, especially in the horse, between the great dorsal and the great oblique of the abdomen, we find a large s.p.a.ce, in which the ribs, with the intercostals which join them, are uncovered; the muscles in question separate the one from the other, under the influence, it would seem, of the great dimensions of the lateral wall of the thorax.
[Ill.u.s.tration: FIG. 6.--A VERTICAL SECTION OF THE THORAX OF A QUADRUPED (DIAGRAMMATIC).
1, Fifth dorsal vertebra; 2, sternal region; 3, costal region of one side; 3", costal region of the opposite side.]
=The Sternum.=--The sternum is, in quadrupeds, directed obliquely downwards and backwards; its form varies in different species. In the carnivora, it consists of eight bones, irregularly cylindrical in form, being slightly flattened from within outwards, and thickened at their extremities. They remain separate, and this contributes elasticity and flexibility to the thorax. The first nine costal cartilages articulate directly with the sternum. The first of these cartilages articulates with a nodule situated a little above the middle of the first bone of the sternum.
In the horse the sternum is flattened laterally in its anterior portion, and from above downwards in its posterior half. The six bones which form the sternum are connected by cartilage. The keel-shaped piece, situated in front of the sternum, is also cartilaginous. This process, but slightly marked posteriorly, becomes more and more prominent in front, and terminates at its anterior extremity by a prolongation, slightly curved backwards, which projects for some centimetres beyond the cavity in which the first costal cartilage is received. This process is known as the _tracheal process_, or _rostral cartilage_. The posterior extremity of the sternum, flattened from above downwards, ends in a cartilaginous plate; concave superiorly, and convex inferiorly: this is the abdominal prolongation, or _xiphoid appendix_.
In the ox, the sternum is formed of two distinct bones, which are united by an articulation. One, the anterior, is short, and forms the first portion of the sternum; it is slightly flattened from side to side, and vertical in direction. The other, the posterior, is longer, and is formed by the fusion of several small bones; it is placed horizontally, and is flattened from above downwards. At the level of articulation of these two portions, and because of their different directions, the bone is bent. This bend occurs at the point of articulation of the second costal cartilage. On the superior border of the anterior segment the cartilage of the first rib is articulated. The xiphoid appendix, which is cartilaginous, is attached to the extremity of a long process of the last bone of the sternum.
The shape of the anterior extremity of the sternum is influenced by the presence or absence of clavicles. We have seen that in some quadrupeds the clavicles are wanting. In the first case, this extremity is large, and approaches in shape to the corresponding part of the human sternum, which is so clearly designed to give a point of support to the anterior bone of the shoulder. In the second, on the contrary, this extremity is narrow.
The sternum in birds is very different from that in mammalia, which we have been studying. It varies greatly in extent and shape, under the influence of certain conditions. To understand the cause of these variations it is necessary to remember that in man (as, indeed, in other animals; but the example of man, for that which follows, will be more striking, on account of the mobility of his upper limbs) the sternum gives origin to the pectoral muscles, and that these muscles are inserted into other parts of the thoracic limbs, designed by their contraction to draw the arms downwards, forwards, and inwards--that is, when these are in a state of abduction and in a horizontal direction, they draw them towards the anterior surface of the thorax and downwards.
Now, this movement is similar to that made by birds during flight. It is necessary to add that, in the latter case, the more the displacement of the upper limbs has of force and extent, the more the pectoral muscles are developed.
For these reasons, birds, in which, during flight, the movements of the thoracic limbs--the wings--are necessarily energetic, present a great development of the pectoral muscles; having consequently, because an extent of surface for the origin of the muscles commensurate with their development is necessary, a very large and peculiarly shaped sternum (Figs. 18, 6; and 21, 6). Indeed, not only is the sternum large, but, further, in order to form a deeper surface, proportionately adapted to the muscles which arise from and cover it, its anterior surface presents, in the median line, a prominent crest known as the _keel_.
This prominence forms two lateral fossae. We cite as examples, the sternum of the eagle, the vulture, the falcon, and the hawk.
All birds are not, however, equally adapted to flight, for in the domestic c.o.c.k, which flies but a short distance, and badly, the sternum is less developed (Fig. 7); it is also diminished by slots, which diminish its surface. These slots, two on each side, are called from their position the internal and external slots. They are bounded by narrow, elongated, bony processes, an internal and an external; the expanded lower extremity of the latter overlaps the last inferior ribs (see p. 19). The part of the external border which surmounts this external process is hollowed out into grooves, which receive the inferior ribs, and terminates superiorly in an osseous projection known as _the costal prominence_.
In the ostrich, the ca.s.sowary, and the apteryx, which run, but do not fly, the sternum has the form of a plate of bone slightly convex, but without a keel.
[Ill.u.s.tration: FIG. 7.--STERNUM OF A BIRD (THE c.o.c.k): LEFT SIDE, EXTERNAL SURFACE.
1, Keel; 2, internal slot; 3, external slot; 4, internal process; 5, external process; 6, inferior ribs; 7, costal process; 8, surface for articulation with the coracoid bone.]
The shape of the sternum, correlated to the faculty of flight (or of swimming; apropos of which we may cite the penguin, of which the rudimentary wings resemble fins, and perform their functions only), or the absence of this faculty, has furnished the division of birds into two groups. In one are included, under the name _Carinates_ (_carina_, keel), those in which the sternum is provided with a keel; in the other division are those in which the sternum is not furnished with one. These latter, on account of their unique mode of progression, are more nearly allied to the mammals.
The keel is developed in flying mammals (bats).
=Ribs and Costal Cartilages.=--There are on each side of the thorax as many ribs as there are dorsal vertebrae. In animals, as in man, the ribs which articulate with the sternum by their cartilages are called _true_, or _sternal_ ribs; those whose cartilages do not articulate with the sternum are called _false_, or _asternal_. The longer ribs are those situated in the middle region of the thorax.
The ribs are directed obliquely downwards and backwards, and this obliquity is more marked in the posterior ones than in the anterior.
They are, however, less oblique than in the human being; what proves this is that the first rib in man is oblique, while in quadrupeds it is vertical.
The curvature of the ribs is less p.r.o.nounced in quadrupeds than in the human being, but this is not equal in all animals. The ribs of the bear are more curved than those of the dog; the latter has ribs more curved than those of the horse.
Each rib, at its vertebral extremity, presents, from within outwards, a wedge-shaped head for articulation with two dorsal vertebrae, a neck, and a tuberosity. External to the tubercle are found some rough impressions, for muscular attachments, which correspond to the angle of the human rib.
In the following table, we give the number and cla.s.sification of the ribs of some animals:
NUMBER OF THE RIBS ON EACH SIDE OF THE THORAX.
Sternal. Asternal.
Bear 14 divided into 9 and 5 Dog 13 " " 9 " 4 Cat 13 " " 9 " 4 Rabbit 12 " " 7 " 5 Pig 14 " " 7 " 7 Horse 18 " " 8 " 10 Camel 12 " " 8 " 4 Ox 13 " " 8 " 5 Sheep 13 " " 8 " 5
The costal cartilages, by which the first ribs are united to the sternum (sternal ribs), whilst the latter are united one to the other without being directly connected with the sternum (asternal ribs), are, as a rule, in quadrupeds, directed obliquely downwards, forwards, and inwards; each forms, with the rib to which it belongs, an obtuse angle more or less open anteriorly. Their length is proportionate to that of the ribs. The cartilages, which are continued from the asternal ribs, unite and form the borders, directed obliquely downwards and forwards, of the fossa which is found at the inferior and posterior part of the thorax, and which forms the lateral limits of the epigastric region. In the dog and cat the ribs are thick and almost cylindrical; the costal cartilages are thicker at the margin of the sternum than at their costal extremity. In the ox, the ribs are flattened laterally and are very broad, the more so as we examine a portion further from the vertebral column. From the second to the twelfth they are quadrangular in the superior fourth, and thicker than in the rest of their extent. The first costal cartilage is vertical; the following ones are progressively more oblique in a direction downwards and forwards. The four or five cartilages which succeed the first unite with slight obliquity to the sternum; their union with that bone gives the impression of a very strong, well-knit apparatus. The costal cartilages which unite with the sternum are flattened laterally in the portions next the ribs, and flattened from front to back in the rest of their extent.
In the horse the ribs increase in length from the first to the ninth; they are flattened from without inwards, and increase in width from the first to the sixth or seventh, and the following ones become narrower.
The costal cartilages, from the second to the eighth, are, as in the ox, at first flattened laterally, near the ribs; while near the sternum they are flattened from front to back.
In birds, the ribs are each furnished with a flat process (Fig. 18, 10), which springs from the posterior border, is directed backwards, and overlaps the external surface of the succeeding rib. These processes are not found, as a rule, on the first or last ribs.
As for the costal cartilages, they are, as a rule, ossified, and receive the name of inferior ribs (Fig. 18, 11), united to the preceding (superior ribs; Fig. 18, 9) by articulation; by the other extremity they are joined to the sternum; the first superior ribs generally want them.
Sometimes the last inferior rib becomes connected with the one that precedes it, not articulating with the sternum; and thus recalls the relations of the asternal ribs which we have noticed in our study of the mammals.
In the bat, as in birds, the costal cartilages are ossified.
THE ANTERIOR LIMBS[5]
[5] Consult Figs. 21, 33, 34, 38, 39, 46.
The anterior limbs, h.o.m.ologous to the upper limbs in man, are formed, as in the latter, of four segments: the shoulder, the arm, the forearm, and the hand. These limbs, considered in the vertebral series, present themselves under very different aspects, which are determined by the functions they are called upon to perform.
They const.i.tute the forepaw in terrestrial mammals; in aerial vertebrates they form wings; in aqueous mammals they act as paddles. In whatever series we study them, we can readily find the relationship of the different parts; it is very easy to recognise the same bones in the upper limbs of the human being, the wings of the bat (Fig. 8) and of birds (Fig. 21), and in the anterior paddles of the seal (Fig. 9) and of the dolphin.
[Ill.u.s.tration: FIG. 8.--ANTERIOR LIMB OF THE BAT: LEFT SIDE, ANTERIOR SURFACE.
1, Clavicle; 2, scapula; 3, humerus; 4, radius; 5, cubitus; 6, carpus; 7, thumb; 8, metacarpus; 9, phalanges.]
In quadrupeds, the shoulder and arm are hidden, the latter more or less completely, in the muscular ma.s.s which binds it to the lateral wall of the trunk; so that the anterior limbs only present; free from the trunk: the elbow, forearm, and hand.
The Shoulder
In some vertebrates, the shoulder is formed of two bones--the scapula and clavicle; in others of only one bone--the scapula; the clavicle in this case does not exist.
=The Scapula or Omoplate.=--The scapula is situated on the lateral surface of the thorax, and is directed obliquely, from above downwards and from behind forwards.
We must first recall, so as to be able to make a comparison, that in man this bone is placed at the posterior surface of the thoracic cage; so that if we look at the human thorax on one of its lateral aspects we see chiefly the external border of the scapula; it is the external surface (h.o.m.ologous to the posterior surface of the human scapula) which we see in its full extent when we look on the same surface of the thorax in quadrupeds.
[Ill.u.s.tration: FIG. 9.--ANTERIOR LIMB OF THE SEAL: LEFT SIDE, EXTERNAL SURFACE.
1, Scapula; 2, humerus; 3, radius; 4, ulna; 5, carpus; 6, metacarpus; 7, phalanges of the fingers.]