Artistic Anatomy of Animals Part 23

It is with these latter that are blended the long and slender tendons which form in a great measure the two other interosseous muscles previously described.

A ligament of the same kind is found in the ox (Fig. 74, 23).

MUSCLES OF THE POSTERIOR LIMBS

Muscles of the Pelvis

The muscles which specially interest us in this region, because of their superficial position, are the gluteus maximus and the gluteus medius. As for the gluteus minimus, it is deeply situated, and more or less sharply marked off from the second of the preceding muscles.

Inasmuch as the gluteus medius is more simple in arrangement than the maximus, and will aid us in arranging our ideas in connection with the latter, it is with the study of it that we will commence.

=Gluteus Medius= (Fig. 68, 29; Fig. 69, 26; Fig. 70, 35).--This muscle, as in man, occupies the external iliac fossa. But this latter being directed differently in the digitigrades and the ungulates, as we have pointed out in the section on osteology (see pp. 91 and 99), the muscle in question has consequently not the same direction in the two groups of animals, being turned outwards in the first, and upwards in the second.

It is the thickest of the glutei, and gives to the region which it occupies a rounded form.

From the iliac fossa from which it arises the fleshy fibres are directed towards the femur, to be inserted into the great trochanter. It is covered by an aponeurosis, and in part by the great gluteal. It completely covers the small gluteal, which veterinary anatomists designate by the name of the _deep gluteal_.

In the carnivora it does not pa.s.s in front of the iliac crest, but, in the ox, and more particularly in the horse, it is prolonged anteriorly, and thus covers, to a certain extent, the muscles of the common ma.s.s.

When it contracts, taking its fixed point at the pelvis, the gluteus medius extends the thigh, which it is also able to abduct. If, on the other hand, its fixed point is on the femur, it acts on the trunk, which it raises, producing oscillating movements of the pelvis. It contributes in this way to the action of rearing. We also see it distinctly appear by the prominence which it produces in the dog, which, according to the time-honoured phrase, _fait le beau_.

=Gluteus Maximus= (Fig. 68, 28; Fig. 69, 25; Fig. 70, 33, 34).--The great gluteal muscle, further designated in veterinary anatomy the _superficial gluteal_, is proportionately less developed in quadrupeds than in man. Indeed, in the latter, where it is of very great thickness, its volume is due to the important function which it fulfils in maintaining the biped att.i.tude.

In quadrupeds it contributes to form the superficial part of the crupper and the external surface of the thigh. It is divided into two parts: one anterior, the other posterior.

With regard to this latter, it will be necessary to indicate how it has been sometimes regarded, and to what portion of the muscular system in man it corresponds. But we believe that it is better to see beforehand, without any preconceived idea, how these two parts are arranged.

In the dog, the anterior portion of the gluteus maximus arises from the sacrum, while some fibres situated further forward arise from the surface of the gluteus medius, near the iliac spine, and from the tensor of the fascia lata with which these fibres are blended. The posterior portion, united to the preceding--that is to say, to those of its fibres which arise from the sacrum--takes its origin from the first coccygeal vertebra. These two portions are directed towards the femur, to be inserted into the great trochanter, and to the external branch of the superior bifurcation of the linea aspera.

In the cat, the posterior bundle is less definitely blended with the anterior. By a long and slender tendon which, behind, turns around the great trochanter, and pa.s.ses along the surface of the fascia lata, it proceeds to join the knee-cap.

In the pig, the posterior portion is much more developed.

In the horse, the anterior portion arises from the internal iliac spine (posterior in man), from the external iliac spine (anterior in man), and, between these two osseous points, from the aponeurosis, which covers the gluteus medius. Between these two origins the muscle is deeply grooved, so that the tendency is to divide into two portions, each of which is directed towards one of the iliac angles. In this groove the gluteus medius is to be seen.

The fleshy bundles converge, and are directed towards the external aspect of the femur, to be inserted into the osseous prominence known as the third trochanter, after pa.s.sing beneath the fleshy fibres of the posterior portion. The latter, which is more considerable than the preceding portion, arises above from the sacral crest, from the aponeurosis which envelops the coccygeal muscles, from the sacro-sciatic ligament, and from the tuberosity of the ischium. From this origin it pa.s.ses downwards, expands, then, describing a curve with the convexity behind, it becomes narrowed, and proceeds to be inserted by a deep fasciculus into the third trochanter, to the fascia lata, and, lastly, to the knee-cap by the inferior part of its tendon.

Above, its posterior border is covered by the semi-tendinosus; interiorly, the same border is in relation with the biceps femoris.

In the ox, the two parts of the great gluteal muscle are blended together.

The long and broad fleshy band which they form arises in a manner corresponding to that which we have just indicated in connection with the horse, except that it has no attachment to the femur. The fascia lata adheres strongly to its anterior border for a considerable length.

The form of the superior border of the great gluteal muscle of this animal differs from that of the a.n.a.logous portion in the horse. This difference results from the peculiar aspect which the corresponding region of the pelvis presents, and from the fact that, in the ox, as the semi-tendinosus does not cover the portion of the great gluteal which arises from the tuberosity of the ischium, the attachments of this muscle to the sacro-sciatic ligament are uncovered.

Its descending portion, as a whole, has a rectilinear form, and does not form a curve such as we indicated in the case of the horse.

The anterior portion of the great gluteal flexes the thigh. As regards the posterior portion, it extends the thigh, and abducts it.

The action of this latter portion is particularly interesting as regards the horse, because of the great development of the muscular ma.s.s which this region presents in this animal. If the muscle takes its fixed point above, it acts, in the extension of the thigh during walking, by projecting the trunk forward during the whole time that the hind-limb to which it belongs is in contact with the ground. If, on the contrary, it takes its fixed point below, it makes the pelvis describe a see-saw movement, upwards and backwards, on the c.o.xo-femoral articulation, and so contributes to the action of rearing.

Now that we have a knowledge of the disposition of the great gluteal muscle, the moment has come to inquire what is the signification of its posterior portion. The action of the anterior part being clearly comparable to the human great gluteal, there can be no doubt as regards the h.o.m.ology of this portion, so we will not insist on it further.

Of the posterior portion it is wholly different, for it is the h.o.m.ologue of a fleshy bundle annexed to the great gluteal of man, but which is not developed except as an abnormality.

Indeed, we sometimes find, placed along the inferior border of the great gluteal, a fleshy fasciculus, separated from this muscle by a slight inters.p.a.ce. This fasciculus, long and narrow, takes origin from the summit of the sacrum, or the coccyx, and goes to partake of the femoral insertions of the muscle which it accompanies. We further note a muscle of the same kind, and presenting the same aspect, which comes from the tuberosity of the ischium. Notwithstanding the difference which exists, it is this abnormal fasciculus of man which in the quadrupeds here studied is considered as const.i.tuting the posterior portion of the great gluteal.

Bourgelat, considering this posterior portion as belonging to the biceps cruris, to which, it is true, it adheres, forms of them a muscle which he designates under the name of the _long vastus_. The anterior fasciculus of this long vastus is none other than the posterior portion of the great gluteal which we have just been studying.

Muscles of the Thigh

These muscles are divided into three regions: posterior, anterior, and internal.

In a corresponding manner to that which we described in connection with the arm, the thigh is applied to the side of the trunk, and is free, more or less, only at the level of the inferior part.

Further, by reason of this shortening of the femur, the great gluteal muscle, which is elongated in the ox and the horse, for example, occupies in part the region corresponding to that which in man is occupied by the muscles of the thigh, which here are reduced in length.

In other words, they are not superposed, as in the human species, but juxtaposed. This is what we will verify further on.

The thigh, as a whole, is flattened from without inwards, its transverse diameter being less in extent than its antero-posterior. Its external surface is slightly rounded; that is, of course, in quadrupeds with sufficiently well-developed muscles. Its internal surface is known as the _flat of the thigh_.

Muscles of the Posterior Region

It is not unprofitable to recall to mind what muscles form the superficial layer of this region in the human being. They are the biceps cruris, semi-tendinosus, and semi-membranosus. We now proceed to discover their a.n.a.logues in quadrupeds.

=Biceps Cruris= (Fig. 68, 30; Fig. 69, 27; Fig. 70, 36).--It is this which, according to Bourgelat, forms the central and posterior portions of the long vastus muscle which we have mentioned above.

We know that the biceps of man is so named from the two portions which form its upper part. In domestic quadrupeds, and also in the majority of the mammals, this muscle is reduced to a single portion, that which comes from the pelvis. It is therefore the portion which arises from the femur which does not exist. This condition is sometimes found as an abnormality in the human species.

The biceps arises from the tuberosity of the ischium; hence it is directed, widening as it goes, towards the leg, where it terminates by an aponeurosis which blends with the fascia lata and the aponeurosis of the leg, and then proceeds to be attached to the anterior border or crest of the tibia. By its inferior portion it limits externally the posterior region of the knee--the popliteal s.p.a.ce.

A fibrous intersection traverses the biceps in its whole length, with the result that the muscle looks as if formed of two portions, one of which is situated in front of the other.

In the dog and the cat it also arises from the sacro-sciatic ligament.

At this level its contour is distinguishable from that which corresponds to the gluteal muscles, so that we there find two prominences one above the other. The superior is formed by the gluteal muscles; the inferior corresponds to the tuberosity of the ischium. The two prominences are separated by a depression, from which the biceps emerges. We draw attention to this form, the character of which is so expressive of energy in the carnivora.

In these animals the biceps is inserted, by its anterior fibres, into the articulation of the knee, while in the rest of its extent it covers in great measure by its aponeurosis the external aspect of the leg.

In the pig, the biceps is but slightly marked off from the posterior part of the great gluteal. In the ox, the division between these two muscles is a little more distinct.

In the horse, the sciatic origin of the biceps is covered by the semi-tendinosus, so that it only becomes free lower down, to appear in the s.p.a.ce limited behind by the semi-tendinosus, and in front by the posterior part of the gluteus maximus.

When the biceps contracts, taking its fixed point from above, it flexes the leg and helps to extend the thigh. If, on the other hand, it takes its fixed point from below, it lowers the ischium, makes the pelvis undergo a see-saw movement, and acts thus in the movement of rearing. It is sometimes called, on account of one of its actions, and the position which it occupies, the 'external flexor, or peroneal muscle of the leg.'

=Semi-tendinosus= (Fig. 68, 31; Fig. 70, 37; Fig. 87, 1; Fig. 88, 1; Fig. 89, 28).--This muscle forms the contour of the thigh posteriorly, so that when the latter is viewed from the side, it is the semi-tendinosus above all that forms the outline. But, as we shall soon see, it is in this case more distinct above than below, because of the deviation which it undergoes in order to occupy by its inferior part the internal side of the leg.