Artistic Anatomy of Animals Part 15

It arises by digitations from the external surface of the dorsal vertebrae; from the first eight in the horse, ox, and dog.

[Ill.u.s.tration: FIG. 67.--MYOLOGY OF THE HORSE: INFERIOR ASPECT OF THE TRUNK.

1, Anterior extremity of the sternum; 2, point of the shoulder and inferior portion of the mastoido-humeral muscle; 3, pectoralis major (sterno-humeral); 4, pectoralis major (sterno-aponeurotic); 5, point of the elbow; 6, pectoralis minor (sterno-trochinian); 7, serratus magnus; 8, external oblique; 9, sheath of the rectus abdominis; 10, linea alba; 11, the umbilicus; 12, external oblique divided in order to expose the rectus abdominis; 13, rectus abdominis.]

The muscular bundles, converging as they proceed, towards the scapula, pa.s.s under this bone, to be inserted into the superior portion of the subscapular fossa, near the spinal border. The inferior portion of its posterior digitations is visible in the ox and in the horse; these digitations are less visible in the pig. They are not seen at all in the dog (Fig. 68) or cat, for in these animals the great dorsal muscle covers them completely.

The great serratus muscle, by the position which it occupies and the arrangement that it presents, forms with the corresponding muscle of the opposite side a sort of girth, which supports the thorax, and at the same time helps to fix the scapula against the latter.

When it contracts, in taking its fixed point at the ribs, it draws the superior portion of the scapula downwards and backwards in such a way that this bone has its inferior angle directed forwards and upwards. If it takes its fixed point at the shoulder, it then acts on the ribs, raises them, and so becomes a muscle of inspiration.

Because of the connections of the serratus magnus with the levator anguli scapulae, some authors consider it as united with the latter. But as the latter muscle is visible only in the region of the neck (see p.

157), and as it is separately described in man, we prefer to distinguish them from one another. We shall recall the connections to which we have just made allusion when describing the cervical region.

Muscles of the Abdomen

The abdominal wall is, as in man, formed by four large muscles: the external oblique, the internal oblique, and the transversalis, which form the lateral walls, and the rectus abdominis, situated on each side of the middle line of the abdomen. This latter, because of the general direction of the trunk in quadrupeds, has its superficial surface directed downwards.

The arrangement of these muscles closely corresponds to that which we find in the human species.

=The External Oblique Muscle= (Fig. 67, 8, 12; Fig. 68, 5; Fig. 69, 9; Fig. 70, 10).--This muscle arises, by digitations, from a number of ribs, which varies according to the species, the number of the ribs being itself variable for each of them, as we pointed out in connection with the osteology of the thorax. Indeed, the great oblique arises from the eight or nine posterior ribs in the dog and the ox, and from the thirteen or fourteen posterior in the horse. It is attached, besides, to the dorso-lumbar aponeurosis.

These attachments are arranged in a line directed obliquely upwards and backwards, and the first digitations--that is to say, the most anterior ones--dovetail with the posterior digitations of origin of the great serratus muscle.

The fleshy fibres are directed downwards and backwards, and terminate in an aponeurosis which covers the inferior aspect of the abdomen, and proceeds to form the linea alba by joining with that of the muscle of the opposite side, and also to be inserted into the crural arch.

This aponeurosis of the external oblique is covered by an expansion of elastic fibrous tissue, which doubles it externally, and which is known as the _abdominal tunic_. This latter is further developed as the organs of the digestive apparatus are more voluminous, and their weight, consequently, more considerable. For this reason, in the large herbivora, as the ox and the horse, this tunic is extremely thick, whereas in the pig, cat, and dog it is, on the contrary, reduced to a simple membrane. Indeed, in these latter, the abdominal viscera being less developed, the inferior wall of the abdomen does not require so strong a fibrous apparatus for supporting them. The great oblique, when it contracts, compresses the abdominal viscera in all circ.u.mstances under which this compression is necessary; it also acts as a flexor of the vertebral column.

=The Internal Oblique Muscle.=--This muscle, which is covered by the preceding, arises from the anterior superior iliac spine (external angle in ruminants and solipeds) and the neighbouring parts. From this origin its muscular fibres, the general direction of which is opposite to that of the fibres of the external oblique, diverging, proceed to terminate in an aponeurosis, which contributes to the formation of the _linea alba_, and to be attached superiorly to the internal surface of the last costal cartilages. It has the same action as the great oblique. What it presents of special interest is the detail of form which it determines in the region of the flank; this detail is _the cord of the flank_. It is characterized by an elongated prominence which, starting from the iliac spine, is directed obliquely downwards and forwards, to terminate near the cartilaginous border of the false ribs.

Often very apparent in the ox, and still more so in the cow, the cord in question contrasts with the depression which surmounts it; this depression is situated below the costiform processes of the lumbar vertebrae, and is called the _hollow of the flank_. It is so much the more marked as the ma.s.s of the intestinal viscera is of greater weight.

We sometimes meet with a case of the presence of this hollow in the horse. But when in the latter, the flank is well formed, the hollow is scarcely visible, and the cord but slightly prominent. It is only in emaciated subjects that these details are found clearly marked.

=Transversalis Abdominis.=--This muscle being deeply situated does not present any interest for us. We will, however, point out, in order to complete the series of muscles which form the abdominal wall, that the direction of its fibres is transverse, and that they extend from the internal surface of the cartilages of the false ribs, and the costiform processes of the lumbar vertebrae to the _linea alba_.

=The Rectus Abdominis= (Fig. 67, 13; Fig. 68, 6).--This muscle, enclosed, as it is in man, in a fibrous sheath (Fig. 67, 9) formed by the aponeuroses of the lateral muscles of the abdomen, is a long and wide fleshy band, which, as in the human species, reaches from the thorax to the pubis.

What distinguishes it in quadrupeds is that there are costal attachments which extend further on the sternal surface of the thorax, and the number of its aponeurotic insertions, which, in general, is more considerable. These are, indeed, six or seven in number in the pig and in ruminants, and about ten in the horse.

It is true that we may find but three in the cat and dog; still, we often find as many as six. These intersections are not marked on their exterior by transverse grooves, such as we find in the human species in individuals with delicate skin and whose adipose tissue is not very much developed.

The rectus abdominis is covered, in its anterior portion, by the sterno-trochinian muscle (posterior segment of the small pectoral). In contracting, this muscle brings the chest nearer the pelvis, and as a result flexes the vertebral column. It also contributes to the compression of the abdominal viscera.

=Pyramidalis Abdominis.=--This unimportant little muscle, which in man is situated at the lower part of the abdomen, extends from the pubis to the _linea alba_. It is not present in the domestic animals.

We consider it interesting, however, to point out, although the fact is not a very useful one as regards external form, that this muscle is distinctly developed in marsupials.

We know that in the opossum, the kangaroo, and the phalanger fox, the young are brought forth in an entirely incomplete state of development, and that, during a certain period, they are obliged to lodge in a pouch which is placed at the lower part of the abdomen of the mother. Now, this pouch contains the mammary glands; but the young, being too feeble to exercise the requisite suction, the pyramidal muscles come to their a.s.sistance. These muscles, in contracting, approximate to one another two bones which are placed above the pubis, the (so-called) marsupial bones (see Fig. 80); by their approximation the bones in question, which are placed behind and on the outer side of the mammary glands, compress the latter, and thus is brought about the result which the little ones, on account of their feebleness, would, without that intervention, be incapable of obtaining for themselves.

Muscles of the Back

=Trapezius= (Fig. 68, 1, 2; Fig. 69, 1, 2; Fig. 70, 1, 2).--This muscle, more or less well developed, according to the species, is divided into two portions, of which the names indicate the respective situations--a cervical and a dorsal.

These two parts, considered in the order in which we find them, take their origin from the superior cervical ligament and from the spinous processes of the first dorsal vertebrae. From these different points the fibres are directed towards the shoulder; the anterior are, consequently, oblique downwards and backwards, and the posterior are directed downwards and forwards. They are inserted into the scapula in the following manner: the fibres of the dorsal portion are attached to the tuberosity of the spine; those of the cervical region are also fixed into the same spine, but into a considerably larger surface.

The cervical portion occupies, in the region of the neck, an area relatively smaller than the corresponding portion of the trapezius in man. This diminished degree of development results from the absence, complete, or nearly so, of the clavicle in the animals which we are now considering. We remember, that the trapezius of man is partly inserted into the clavicle, and the disappearance of this latter cannot fail to bring modifications in the general disposition of the corresponding portion of the muscle. There results a disconnection of this latter, and it becomes united to other muscular fibres to form a muscle with which we shall soon have to deal--the mastoido-humeral (see p. 150).

As specific differences we should add that the trapezius occupies a more or less extensive portion of the median and superior regions of the neck; terminating at a considerable distance from the head in the dog and horse, it, on the contrary, approaches it in the pig and in ruminants. The cervical portion, when it contracts, draws the scapula upwards and forwards, the dorsal portion draws it upwards and backwards.

When the trapezius acts as a whole the scapula is raised.

[Ill.u.s.tration: FIG. 68.--MYOLOGY OF THE DOG: SUPERFICIAL LAYER OF MUSCLES.

1, Trapezius, cervical portion; 2, trapezius, dorsal portion; 3, superior outline of the scapula; 4, latissimus dorsi; 5, external oblique muscle; 6, rectus abdominis; 7, pectoralis major of the right side; 8, pectoralis minor (sterno-trochinian); 9, 9, mastoido humeral muscle; 10, tendinous intersection, at the level of which is found a rudimentary clavicle; 11, sterno-mastoid muscle; 12, infrahyoid muscles; 13, omo-tracheal or acromio-tracheal muscle; 14, splenius; 15, levator anguli scapulae; 16, deltoid muscle, spinal portion; 17, deltoid, acromial portion; 18, superior extremity of the humerus; 19, supraspinatus; 20, infraspinatus; 21, biceps cubiti; 22, brachialis anticus; 23, triceps cubiti, long head; 24, triceps cubiti, external head; 25, olecranon process; 26, radialis (anterior extensor of the metacarpus); 27, iliac crest; 28, gluteus maximus; 29, gluteus medius; 30, biceps cruris; 31, semitendinosus; 32, semi-membranosus; 33, gastrocnemius; 34, tensor of the fascia lata; 35, sartorius; 36, fascia lata drawn up by the triceps; 37, the patella or knee-cap; 38, ischio-coccygeal muscle; 39, superior sacro-coccygeal; 40, lateral sacro-coccygeal; 41, inferior sacro-coccygeal.]

=The Latissimus Dorsi= (Fig. 68, 4; Fig. 69, 5; Fig. 70, 5).--This muscle arises by an aponeurosis, the so-called dorso-lumbar aponeurosis, from the spinous processes of the last dorsal vertebrae (the seven last in the dog, fourteen or fifteen last in the horse), from the spinous processes of the lumbar vertebrae, and from the last ribs. Its fleshy fibres are directed downwards and forwards, being more oblique in direction posteriorly, and pa.s.s on the inner side of the posterior muscular ma.s.s of the arm, to be inserted into the internal lip of the bicipital groove of the humerus, or, a little lower down, on the median portion of the internal surface of the same bone. This latter mode of insertion is met with in the horse and the ox.

The anterior fibres cover the posterior angle of the scapula (as in man, where the corresponding angle, but in this case inferior, is covered by the same muscle), and, a little higher up, are in their turn concealed by a portion of the dorsal fibres of the trapezius. It covers, to a greater or less extent, the great serratus muscle. These relations are similar to those found in the human species.

We find that the fleshy fibres of the great dorsal are prolonged more or less backwards if we examine this muscle in the dog, the ox, the pig, and the horse. Indeed, the fibres reach to the thirteenth rib in the dog and the cat (that is to say, the last rib), the eleventh in the ox, tenth in the pig, and twelfth only in the horse. We say 'only' in connection with this last because it is necessary to remember that the ribs are eighteen in number on each side of the thorax of this animal, and that, accordingly, the fleshy fibres of the great dorsal muscle are, relatively, of small extent.

When this muscle contracts it flexes the humerus upon the scapula, and helps to draw the whole of the anterior limb backwards and upwards.

There is a muscular fasciculus which, because of its relations with the muscle we have just been studying, is known as the _supplementary muscle of the latissimus dorsi_. But as, on the other hand, this fasciculus is in relation with the triceps, we shall in preference consider it in relation with this latter (see p. 173).

[Ill.u.s.tration: FIG. 69.--MYOLOGY OF THE OX: SUPERFICIAL LAYER OF MUSCLES.

1, Trapezius, cervical portion; 2, trapezius, dorsal portion; 3, outline of the scapula; 4, spine of the scapula; 5, latissimus dorsi; 6, small posterior serratus; 7, prominence caused by the costiform processes of the lumbar vertebrae; 8, serratus magnus; 9, external oblique; 10, pectoralis major (sterno-humeral); 11, mastoido-humeralis; 12, atlas; 13, atlas; 14, parotid gland; 15, sterno-mastoid muscle; 16, infrahyoid muscles; 17, omo-trachelian or acromio-trachelian muscle; 18, deltoid; 19, brachialis anticus; 20, triceps, long head; 21, triceps, external head; 22, olecranon; 23, radialis (anterior extensor of the metacarpus); 24, anterior iliac spine; 25, gluteus maximus; 26, gluteus medius; 27, biceps cruris; 28, semitendinosus; 29, gastrocnemius; 30, tensor of the fascia lata; 31, fascia lata covering the triceps of the thigh; 32, patella; 33, ischio-coccygeal muscle; 34, superior ischio-coccygeal; 35, lateral ischio-coccygeal; 36, inferior ischio-coccygeal.]

The aponeurosis by which the great dorsal arises from the vertebral column covers, as in man, the muscles which occupy the grooves situated on each side of the spinous processes--the spinal muscles or common muscular ma.s.s, if we regard them as a whole (Fig. 70, 7); the sacro-lumbar and the long dorsal muscles covering the transverse spinal, if we consider them as distinct.

It would be superfluous to enter here into a detailed examination of these muscles.

If they are but little developed the spinous processes become prominent under the skin; if they are more so they may by their thickness project beyond the level of these processes, and these latter thus come to lie in a groove more or less marked, which, on account of the division which is determined by its presence, has caused the regions which it occupies to be designated by the names _double back_ and _double loins_.

The muscles are extensors of the vertebral column.

Under the aponeurosis of the great dorsal muscle there is found in man another muscle, the serratus posticus inferior, which, on account of being deeply placed and its slight thickness, offers nothing of interest in connection with the study of external form. It arises from the spinous processes of the three last dorsal vertebrae and those of the three first lumbar; it then pa.s.ses upwards and outwards, and divides into four digitations, to be inserted into the inferior borders of the four last ribs. We repeat that it is covered by the great dorsal muscle.

In the pig, ox, and horse, which have this latter muscle less developed in its posterior portion, the same small serratus muscle, known as the _posterior serratus_, is visible in the superficial layer of muscles (Fig. 69, 6; Fig. 70, 6). The number of its digitations is more or less considerable according to the species examined.

=The Rhomboid Muscle= (Fig. 70, 21).--In order to make intelligible the position of the rhomboid in the superficial layer in quadrupeds, it appears to us necessary to recall the anatomical characters of the muscle as found in man. The rhomboid arises from the inferior portion of the posterior cervical ligament, from the spinous process of the seventh cervical vertebrae and the four or five upper dorsal; thence pa.s.sing obliquely downwards and outwards, it is inserted into the spinal border of the scapula, into the portion of this border which is situated below the spine; it sometimes extends to the middle of the interval which separates this latter from the superior internal angle of the same bone.

The portion of the muscle which arises from the cervical ligament and the seventh cervical vertebra is often separated from the lower portion by a cellular inters.p.a.ce. For this cause some anatomists have described the rhomboid as consisting of two parts--the superior or small rhomboid and the inferior or large rhomboid, on account of the position occupied by each, and of their difference in volume.

This muscle can only be seen in the region of the back, in the s.p.a.ce limited externally by the spinal border of the scapula, below by the latissimus dorsi, and internally by the trapezius, which covers it in the rest of its extent. It is not in this s.p.a.ce that it is seen in certain quadrupeds. As we pointed out in the section on osteology, the spinal border of the scapula is short, and it seems to be due to this limitation in length that the trapezius and the latissimus dorsi muscle are, at this level, in contact the one with the other in such a way that they fill up the interval in which the rhomboid is seen in man.