Marvel Anatomy

THE TRUNK

The Vertebral Column

We commence the study of the skeleton with a description of the trunk.

Fig. 1.-A Human Skeleton in the Attitude of a Quadruped. To give a general Idea of the position of the Bones in other Vertebrates.

The trunk being, in quadrupeds, horizontal in direction (Fig. 1), the two regions of which it consists occupy, for this reason, the following positions: the thorax occupies the anterior part, the abdomen is placed behind it; the vertebral column is horizontal, and is situated at the superior aspect of the trunk; it projects beyond the latter: anteriorly, to articulate with the skull; and, posteriorly, to form the skeleton of the tail, or caudal appendix.

The number of the vertebr? is not the same in all mammalia. Of the several regions of the vertebral column, the cervical shows the greatest uniformity in the number of the vertebr? of which it consists, with but two exceptions (eight or nine in the three-toed sloth, and six in the manatee); we always find seven cervical vertebr?, whatever the length of the neck of the animal. There are no more than seven vertebr? in the long neck of the giraffe, but they are very long ones; and not less than seven in the very short neck of the dolphin, in which they are reduced to mere plates of bone not thicker than sheets of cardboard. If the cervical region presents uniformity in the number of its bones, it is not so with the other regions of the column.

The following table shows their classification in some animals:

Vertebr?.

Cervical. Dorsal. Lumbar.

Bear 7 14 6

Dog 7 13 7

Cat 7 13 7

Rabbit 7 12 7

Pig 7 14 6 or 7

Horse 7 18 6 or 5

Ass 7 18 5

Camel 7 12 7

Giraffe 7 14 5

Ox 7 13 6

Sheep 7 13 6

It is worthy of notice that in birds the number of the cervical vertebr? is not constant, as in mammals; they are more numerous than the dorsal. These latter are almost always joined to one another by a fusion of their spinous processes; the two or three last vertebr? are similarly united to the iliac bones, between which they are fixed. The dorsal vertebr? thus form one piece, which gives solidity to the trunk, and provides a base of support to the wings, for the movements of flying. There are, so to speak, no lumbar vertebr?, the bones of that region, which cannot be differentiated from the sacrum, having coalesced with the bones of the pelvis.

Vertebr?.

Cervical. Dorsal.

Vulture 15 7

Eagle 13 9

Cock 14 7

Ostrich 18 9

Swan 23 10

Goose 18 9

Duck 15 9

In reptiles, the relation between the number of the cervical vertebr? and that of the dorsal is very variable; some serpents are devoid of cervical vertebr?, having only dorsal ones-that is, vertebr? carrying well-developed ribs.

Vertebr?.

Cervical. Dorsal. Lumbar.

Crocodile 7 14 3

Caiman 7 12 5

Boa 3 248 0

Python 0 320 0

Viper 2 145 0

Fig. 2.-Size of the Atlas compared with the Transverse Dimensions of the Corresponding Parts of the Skull in Man.

1, Atlas; 2, mastoid process; 3, external occipital protuberance; 4, inferior maxilla.

Regarding the direction of the vertebral column in animals, in which the trunk is not vertical, it is evident that the spinous processes point upward, and that in comparing them with those of man they must be arranged so that the superior surface of the human vertebra will correspond to the anterior surface of that of the quadruped. Of the cervical vertebr?, the atlas and axis call for special notice. Apropos of the atlas, we find that it, in the human being, is narrower than the corresponding parts of the skull, and is therefore hidden under the base of the cranium (Fig. 2); in quadrupeds its width is equal to that of the skull, and sometimes exceeds, because of the great development of its wing-shaped transverse processes, that of the neighbouring parts of the head (Fig. 3). On this account those transverse processes often project under the skin of the lateral surfaces of the upper part of the neck.

Fig. 3.-Size of the Atlas compared with the Transverse Dimensions of the corresponding Regions of the Skull in a Dog.

1, Atlas; 2, zygomatic arch; 3, external occipital protuberance; 4, inferior maxilla.

The axis is furnished on its anterior surface with the odontoid process, which articulates with the anterior (or inferior) arch of the atlas, according to the direction of the neck. The spinous process, flattened from without inwards, is more or less pointed; it is elongated from before backwards, so as partly to overlap the atlas and the third cervical vertebra.

We find that this process overlaps less and less the neighbouring vertebr? when we examine in succession the bear, the cat, the dog, the ox, and the horse. With regard to the other vertebr? of this region, they diminish in width from the second to the seventh; and, in some animals, the anterior surface of the body presents a tubercle which articulates with a cavity hollowed in the posterior surface of that of the vertebra before it; this feature dwindles away in the dorsal and lumbar regions.

The spinous process, slightly developed in the third cervical vertebra, gradually increases in size to the seventh, the spinous process of which, long and pointed, well deserves the name of the prominent which is bestowed on it; but it should not be forgotten that the spinous process of the axis is equally developed.

On the inferior surface of the body of each of the vertebr? is found a prominent crest, especially well marked at the posterior part; this crest is but slightly developed in the bear and in the cat tribe, and is not found in swine.

The transverse processes of the cervical vertebr?, from their relation to the trachea, are known as the tracheal processes.

The most marked characteristic of the dorsal vertebr? is furnished by the spinous processes. They are long and narrow. As a rule, the spinous processes of the foremost dorsal vertebr? are the most developed and are directed obliquely upwards and backwards. As we approach the last vertebr? of this region, the processes become shorter and tend to become vertical, and the last ones are even, in some cases, directed upwards and forwards; this disposition is well marked in the dog and the cat. In the cetaceans, on the contrary, the length of the spinous processes increases from the first to the last.

In the horse the spinous processes of the first dorsal vertebr? produce the prominence at the anterior limit of the trunk, where the mane ends, which is known as the withers.

Fig. 4.-Lumbar Vertebr? of a Quadruped (the Horse): Superior Surface.

1, Spinous process; 2, anterior articular process and transverse process of the first lumbar vertebra of the left side; 3, costiform process.

The lumbar vertebr? are thicker than the preceding; they are known by their short and latterly-flattened spinous processes, and still more readily by their transverse processes, which, as they are evidently atrophied ribs, it is more accurate to denominate costiform processes (Fig. 4). These are long, flattened from above downwards, and directed outwards and forwards.

The true transverse processes are represented by tubercles situated on the superior borders of the articular processes of each of the vertebr? of the lumbar region. Apropos of these different osseous processes, we are reminded that they are also present in the human skeleton.

In the horse the costiform processes of the fifth and sixth lumbar vertebr? articulate, and are sometimes ankylosed, one with the other; the terminal ones articulate with the base of the sacrum. Sometimes the processes of the fourth and fifth are thus related; this is the case in the figure (4) given; here the costiform processes of the fourth and fifth vertebr? articulate, and the two terminal ones have coalesced.

In the ox, the same processes are more developed than in the horse; their summits elevating the skin, produce, especially in animals which have not much flesh, prominences which limit the flanks in the superior aspect. The costiform processes of the last lumbar vertebr? are separate from each other; those of the latter are not in contact with the sacrum.

The Sacrum.[3]-This bone, single and median, is formed by the mutual coalescence of several vertebr?, which vary in number according to the species observed.

[3] In human anatomy, the sacrum and the coccyx are studied as part of the pelvis; we, therefore, in the study of the artistic anatomy of man, study these bones with the bones of the lower limbs. Here we do not follow this plan. In animals the sacrum and the coccyx, as a matter of fact, clearly continue the superior border of the skeleton of the trunk; hence we study them with the vertebral column.

Vertebr? Constituting the Sacrum.-Bears, 5; dogs, 3; cats, 3; rabbits, 4; swine, 4; horses, 5; camels, 4; oxen, 5; sheep, 4.

The sacrum is situated between the two iliac bones; with which it articulates, and contributes to the formation of the pelvis. It is obliquely placed, from before backwards, and from below upwards; immediately behind the lumbar section of the vertebral column; and is continued by the coccygeal vertebr?, which form the skeleton of the tail.

It is triangular in outline, and is generally more narrow in proportion than in the human being. All things considered, it is more large and massive, and of greater density, in species which sometimes assume the upright posture, rather than in those which cannot assume that attitude; for example, the sacrum of the ape, of the bear, of the dog, and of the opossum are proportionately larger than those of the horse.[4]

[4] This is particularly striking only in those portions of the sacrum that are not in relation with the other bones of the pelvis. We think that the general form of this bone depends on the mode of its connexions with the iliac bones and the extent of the articular surfaces by which it is in contact with the latter.

Its superior surface presents a crest, formed by the fusion of the spinous processes of the vertebr? which form it. In certain species these processes are attached only by their bases, and are separated from each other superiorly. In the pig they are wholly wanting.

The Coccygeal Vertebr?.-These vertebr?, few in number (and sometimes ankylosed) in the human being, form in the latter a small series, the coccyx; which is inclined forwards, that is to say, towards the interior of the pelvis. In quadrupeds, on the contrary, their number is large; they are not ankylosed, and they form the skeleton of the caudal appendix.

The first coccygeal vertebr?-that is, those which are next the sacrum-present characters which are common to those of other regions: they have a body, a foramen, and processes. As we trace them backwards, these characters become gradually effaced; and they become little more than small osseous cylinders simply expanded at their extremities.

Direction and Form of the Spinal Column

The curves of the vertebral column are, in quadrupeds, slightly different from those which characterize the human spine. First, instead of their being, as in the latter, curves in the antero-posterior aspect, because of the general attitude of the body, they are turned in the supero-inferior direction.

The cervical region is not a single curve, as in the human being. It presents two: one superior, with its convexity looking upwards; the other inferior, the convexity of which is turned downwards. This arrangement reminds one of that of a console.

The dorsal and lumbar regions are placed in a single curved line, more or less concave downwards; so that in the lumbar region there is no curve analogous to that which exists in man; a form which, in the latter, is due to the biped attitude-that is to say, the vertical position of the trunk. Briefly, there is in quadrupeds one dorso-lumbar curve; and not both a dorsal and a lumbar, with convexities in opposite directions.

At the extremity of the dorso-lumbar region is the sacrum and the caudal appendix, which describe a curve of which the concavity is directed downwards and forwards.

It is necessary to point out that it is not the curves of the three anterior portions of the spinal column which determine the form of the superior border of the neck and shoulders, and of the same part of the trunk. For the first portion, there is a ligament which surmounts the cervical region, and substitutes its modelling influence for that of the vertebr?. It is the superior cervical ligament, which arises from the spinous process of the first cervical vertebr?, and is inserted into the external occipital protuberance on the upper part of the posterior surface of the skull. The summits of the spinous processes of the vertebr? alone give form to the superior median border of the trunk. In this connection we here repeat that it is not the general curvature of the vertebral column which produces the withers, but the great length of the spinous process of the first vertebr? of the dorsal region.

The Thorax

The dorsal vertebr? 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.

Fig. 5.-A Transverse Section of the Thorax of a Man placed Vertically-that is to say, in the Direction which it would assume in a Man placed in the Attitude 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.

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).

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.

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 space, 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.

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 foss?. We cite as examples, the sternum of the eagle, the vulture, the falcon, and the hawk.

Fig. 7.-Sternum of a Bird (the Cock): 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.

All birds are not, however, equally adapted to flight, for in the domestic cock, 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 cassowary, 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.

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 vertebr?. 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 pronounced 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 vertebr?, 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 classification 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, homologous 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.

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.

They constitute 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.