It is in relation superiorly with the tibialis anticus, and inferiorly with the common extensor of the toes; in the ox, it is in contact with this latter muscle throughout its whole length.

The peroneus longus arises from the external tuberosity of the tibia; towards the middle of the leg it is replaced by a tendon. This proceeds towards the tarsus, but previously it pa.s.ses between the tibia and fibula. In the ox it is situated in front of the coronoid tarsal bone; we recollect that this bone is regarded as representing the inferior extremity of the fibula (see p. 97). Then it pa.s.ses into a groove belonging to the cuboid bone or to the cuboido-scaphoid bone in the ox, traverses obliquely the posterior aspect of the tarsus, and is inserted into the rudimentary bone which represents the first toe; or, if this does not exist, into the innermost of the metatarsal bones.

This muscle is an extensor of the foot. It also rotates it outwards in the animals in which the articulation permits this latter movement.

=Peroneus Brevis= (Fig. 83, 8; Fig. 83, 10; Fig. 84, 10; Fig. 86, 6).--In the dog and the cat, this muscle is covered in part by the peroneus longus, and arises from the inferior half of the tibia and the fibula; at the level of the tarsus it becomes tendinous, pa.s.ses into a groove hollowed out on the external surface of the inferior extremity of the fibula, and terminates on the external aspect of the superior extremity of the fifth metatarsal. A little before this insertion it crosses the tendon of the long peroneal in pa.s.sing to the outer side of the latter.

To the short peroneal muscle is found annexed a very thin fasciculus which lies upon it. This fasciculus arises from beneath the head of the fibula, and is soon replaced by a thin tendon, which, accompanying that of the short peroneal, proceeds towards the foot, after having traversed the groove in the inferior extremity of the fibula; pa.s.ses along by the fifth metatarsal (Fig. 84, 12); blends at the level of the first phalanx of the fifth toe with the corresponding tendon of the long extensor of the toes, and partakes of the insertions of this tendon.

This fasciculus is designated by some authors under the name of the peroneal of the fifth toe, or the proper extensor of the same toe. But what makes still further complications is that other authors regard it as an anterior, or third, peroneal. Now, these names are those which other anatomists have applied to the fasciculus of the anterior tibial, which, in the pig and the ox, is fused in part with the long extensor of the toes. Hence there results a confusion which is truly regrettable.

In brief, we can, without inconvenience, consider it as a fasciculus of the short peroneal muscle.

We sometimes find in man, but abnormally, an arrangement which partly recalls that which we have just indicated. It consists in a duplication of the tendon of the short peroneal, one of the branches of which goes to the fifth metatarsal, and the other to the fifth toe; it is sometimes a single fasciculus which goes to the phalanges of this latter. We have met with examples of these anomalies.[31] In the pig, the short peroneal is situated on the same plane as the long. It consists of two clearly distinct fasciculi, which arise from the fibula. The tendon of the anterior fasciculus proceeds to the great external toe--that is to say, the fourth, of which it is the proper extensor. The posterior fasciculus terminates on the small external toe, the fifth, of which it is in like manner the extensor.

[31] edouard Cuyer, 'Anomalies, Osseous and Muscular' (_Bulletins de la Societe d'Anthropologie_, Paris, 1891).

In the ox, the fleshy fibres of the short peroneal arise from a fibrous band which replaces the fibula, and from the external tuberosity of the tibia. Situated behind the long peroneal and on the same plane, it terminates in a tendon which appears at the level of the inferior part of the leg; it pa.s.ses in front of the canon, and is inserted into the external toe, of which it is the proper extensor.

In the horse, it is the sole representative of the peroneal muscles, and veterinary anatomists have given it the name of _the lateral extensor of the phalanges_.

Its fleshy body arises above from the external lateral ligament of the knee-joint, and from the whole length of the fibula. In the middle third of the leg it is narrowed; lower down it is replaced by a tendon. This is lodged in a groove hollowed on the external surface of the inferior extremity of the tibia; then after pa.s.sing along the external surface of the tarsus, it is directed forward, and proceeds to blend towards the middle of the canon-bone with the tendon of the long extensor of the toes, or anterior extensor of the phalanges, of which it shares the insertions. It extends the phalanges into which it is inserted. It also flexes the foot.

Muscles of the Posterior Region

It will not be unprofitable to recall to mind that, in man, the muscles of this region are arranged in two layers: a superficial layer consisting of the gastrocnemius and soleus, to which is added a muscle of little importance, the plantaris, and a deep layer formed by four muscles--the popliteus, flexor longus digitorum, tibialis posticus, and flexor longus pollicis.

The gastrocnemius and soleus, independent in their upper portion, unite below in a common tendon; they thus form also a triceps muscle, which we designate under the name of the triceps of the leg, or triceps suralis, because it forms the elevation of the calf of the leg (from _sura_, calf).

=Gastrocnemius= (Fig. 83, 9, 11; Fig. 84, 13, 14; Fig. 86, 7, 9; Fig.

88, 13).--The external and internal heads of the gastrocnemius, distinct from one another only in their upper portion, arise from the shaft of the femur, above the condyles, on the borders of the popliteal surface, to a relatively considerable extent in the great quadrupeds.

At this level they are situated in the popliteal region--that is to say, in the s.p.a.ce limited externally by the biceps, and internally by the semi-tendinosus. But as they descend to a rather low level on the leg in quadrupeds, and especially in carnivora, they do not, properly speaking, determine a projection of the calf of the leg. However, they pa.s.s from this region but to be soon continued by a tendon--the tendo-Achillis, which is inserted into the calcaneum.

Now, the region of the tarsus is called by veterinarians _the ham_, the posterior surface of which is angular, because of the oblique direction of the leg with regard to the vertical direction of the metatarsus and the presence of the calcaneum; the prominence which this surface presents has received the name of _the point of the ham_, and the tendon which ends there that of _the cord of the ham_.

But the tendo-Achillis does not alone form this cord. Indeed, as we will soon see, the tendon of the superficial flexor of the toes takes part in its formation.

We may add, with regard to the tendo-Achillis, that it is more clearly perceived as an external feature, because the skin sinks in front of it, as it does in man, over the lateral parts of the region which it occupies.

The gastrocnemius, when it contracts, extends the foot on the leg.

It serves to maintain the tibio-tarsal angle in the standing position, and during walking, to determine the steadying of the hind-limbs, which then, after the fashion of a spring, project the body forward.

By an a.n.a.logous movement they take part in the posterior projection of the hind-limbs in the act of kicking; but they are not the only ones to act in this case, the muscles of the b.u.t.tock and thigh also being brought into play.

=Soleus= (Fig. 83, 10; Fig. 86, 8).--This muscle, much less developed in quadrupeds than in man, does not exist in the dog.

With regard to the soleus in the pig, Professor Lesbre says: 'Meckel denied its existence; we, however, believe that it is united to the external head of the gastrocnemius, its origin being transferred to the femur.'[32]

[32] F. X. Lesbre, 'Essai de Myologie comparee de l'homme et des mammiferes domestiques en vue d'etablir une nomenclature unique et rationelle,' Lyon, 1897, p. 169.

But in animals in which it exists, this muscle, of but little importance, occupies the outer side of the leg. It arises above from the external tuberosity of the tibia, and terminates below in a tendon which is united with that of the gastrocnemius.

The soleus has the same action as these latter.

=Plantaris.=--In quadrupeds this muscle is blended with the superficial flexor of the toes, which we will study afterwards.

=Popliteus= (Fig. 87, 16; Fig. 88, 14).--In man, this muscle, which occupies the posterior surface of the tibia, above the oblique line, is completely covered by the gastrocnemius.

In quadrupeds, where it is more voluminous, it projects internally beyond the gastrocnemius, so that it is seen in the internal and superior part of the region of the superficial layer of muscles, immediately behind the internal surface of the tibia, which, as we know, is subcutaneous.

The popliteus arises from the external surface of the external condyle of the femur. Thence its fibres which diverge pa.s.s to be inserted into the superior part of the posterior surface and of the internal border of the tibia. It is in this latter region that it projects beyond the gastrocnemius, but we may add that there it is more or less covered by the semi-tendinosus.

It flexes the leg, and rotates it forwards.

=Superficial Flexor of the Toes= (Fig. 83, 13, 15; Fig. 84, 17; Fig. 86, 11, 13, 13; Fig. 87, 17; Fig. 88, 15, 18, 18).--In man, the h.o.m.ologue of this muscle is found in the sole of the foot. It is called _the short flexor of the toes_. It arises from the calcaneum, and pa.s.ses to the four outer toes. In quadrupeds, it rises as high as the back of the knee, and is found blended with the plantaris.

Further designated by the name of _the superficial flexor of the phalanges_, covered in part by the gastrocnemius, with which it is in relation for a great part of the course which it traverses, this muscle arises from the posterior surface of the femur, on the external branch of the inferior bifurcation of the linea aspera. In the horse, this origin takes place in a depression situated above the external condyle, in the supracondyloid fossa. Then it accompanies the gastrocnemius, and becomes tendinous where the tendo-Achillis commences. It then winds round the latter in placing itself on its inner side, then on its posterior surface, and reaches the calcaneum. It accordingly contributes, as we have already pointed out, to form the cord of the ham. After having become expanded, and having covered as with a sort of fibrous cap the bone of the heel, it descends behind the metatarsus, and presents there an arrangement a.n.a.logous to that which we pointed out in connection with the superficial flexor of the digits--that is, it contributes to form the _tendon_. This prominence, in the form of a cord, we see behind the canon-bone in solipeds and ruminants. It finally terminates in the same way as the muscle with which we have compared it (see p. 197).

In the horse, its fleshy body is but slightly developed, so that its tendon alone is specially visible in the superficial muscular layer, but in the dog and the cat it is large. Hence it results that its fleshy body appears on each side of the inferior half of the gastrocnemius, and produces an elevation which recalls that which the soleus produces on each side of the same muscles in the human species.

The muscles which follow form, with the popliteus, which we have already studied, the deep layer of the posterior region of the leg.

=Flexor Longus Digitorum= (Fig. 87, 19; Fig. 88, 17).--This muscle, in man, is the only common flexor of the toes belonging to the muscles of the leg.

In comparison with the preceding muscle, it is a deep flexor.

Veterinarians have given it the name of _the oblique flexor of the phalanges_.

Visible on the internal aspect of the superficial layer of the muscles of the leg, this muscle arises above from the posterior surface of the external tuberosity of the tibia, becomes tendinous, pa.s.ses towards the metatarsus, and blends with the tendons of the posterior tibial and the long proper flexor of the great-toe. In the dog and the cat it is blended with this latter only.

=Tibialis Posticus= (Fig. 85, 14; Fig. 86, 12; Fig. 87, 20; Fig. 88, 16).--This muscle arises from the external tuberosity of the tibia, and from the head of the fibula. Thence it pa.s.ses to the tarsus, and terminates in different fashion in carnivora and other quadrupeds.

In the dog and the cat, it is inserted into the ligamentous apparatus of the tarsus, or into the base of the second metatarsal.

In the other quadrupeds with which we are here occupied it is blended with the long proper flexor of the great-toe.

It is accordingly in the carnivora that the mode of termination of the tibialis posticus most nearly resembles that of this same muscle in the human species. From this independence there results a special action.

It is an adductor and internal rotator of the foot.

=Flexor Longus Pollicis= (Fig. 84, 18; Fig. 85, 14; Fig. 86, 12; Fig.

87, 18; Fig. 88, 16).--This muscle, as that in man, is the most external of the deep layer of the leg. It is on the external aspect of the latter we perceive it, between the peroneals and the gastrocnemius or tendo-Achillis.

It arises from the fibula and tibia, and is thence directed towards the tarsus. It unites with the long common flexor of the toes to form with it _the deep flexor of the phalanges_, of which it is the princ.i.p.al fasciculus. We may add that in the dog and the cat the posterior tibial remains independent of this latter, but that in the pig, ox, and horse the posterior tibial is united to the preceding to form with them the deep flexor muscle.

Thus const.i.tuted, the deep flexor goes towards the phalanges, where it terminates as the deep flexor of the digits of the fore-limbs (see p.

197). In animals possessed of a canon it contributes to form the _tendon_ (Fig. 85, 16; Fig. 86, 14, 14; Fig. 88, 19, 19).