Artistic Anatomy of Animals Part 27

Muscles of the Foot

We must remember that on the dorsal surface of the foot in man we find but a single muscle--the dorsalis pedis. The remaining subcutaneous structures of this region consist of the tendons of the anterior muscles of the leg which occupy this dorsal aspect.

=Dorsalis Pedis= (Fig. 84, 19).--Also called the extensor brevis digitorum, the dorsalis pedis muscle is found in all domestic quadrupeds; but its development is so much the less as the number of digits is more reduced.

In the dog and the cat it arises from the calcaneum, and is inserted into the three internal toes (the first toe excepted) by uniting with the corresponding tendons of the common extensor.

In the pig its disposition is a.n.a.logous.

As for the dorsalis pedis of the ox and the horse, it is extremely rudimentary, and occupies the superior part of the canon. It is a small, fleshy body, situated on the anterior surface of the metatarsus, which arises from the calcaneum, whence it pa.s.ses to unite at its inferior extremity with the tendon of the extensor of the phalanges.

As regards the muscles of the sole of the foot, we think it unnecessary to occupy ourselves at any length with them because of their slight importance with regard to external form.

We will only recall that in the median portion of this plantar surface we find in man the short flexor of the toes, which in quadrupeds arises higher up, from the posterior surface of the femur; that it belongs to the muscles of the leg; and that it forms the superficial flexor of the toes, which we have already studied.

We may further add that the suspensory ligament of the fetlock in ruminants and solipeds represents, as in the fore-limbs, the interosseous muscles.

MUSCLES OF THE HEAD

We will divide these muscles into two categories: masticatory and cutaneous.

Masticatory Muscles

The muscles of this group which specially interest us are the ma.s.seter and the temporal. As regards the pterygoids, since they are situated within the borders of the inferior maxillary bone, and consequently do not reach the surface, we shall not require to occupy ourselves with them here.

=Ma.s.seter= (Fig. 89, 2; Fig. 90, 1; Figs. 91, 92).--For those who have studied the ma.s.seter of man, it is not difficult to recognise that of quadrupeds. Nevertheless, the particular aspect which it presents in different species gives to its study a certain interest.

Arising from the zygomatic arch, and pa.s.sing downwards and backwards, it is inserted into the external surface of the ramus of the mandible and into its angle.

Its posterior border is in relation with the parotid gland (Fig. 90, 14; Figs. 91, 92), this gland being situated between the corresponding border of the lower jaw bone and the transverse process of the atlas.

Such are the general characters; the following are the particular ones:

In the carnivora it is thick and convex. In the horse it is flat, but more expanded; it forms the _flat of the cheek_. In the ox it is flat, as in the latter; but, while being less thick, it is more prolonged in the vertical direction.

The form of the osseous parts which give it origin is, besides, in relation with these differences, and explains the peculiar characters which the ma.s.seter presents.

[Ill.u.s.tration: FIG. 89.--MYOLOGY OF THE DOG: MASTICATORY MUSCLES (A DEEPER DISSECTION THAN THAT SHOWN IN FIG. 90).

1, Zygomatic arch; 2, ma.s.seter; 3, temporal exposed by the suppression of the auricular and occipital muscles and the pinna of the ear; 4, auditory ca.n.a.l; 5, inferior maxillary bone; 6, digastric.]

Indeed, in the dog and the cat the zygomatic arch, strongly convex, springs up in a marked manner from the plane of the lateral aspects of the skull.

In the horse the same arch, less prominent externally, is prolonged by a rectilinear crest on the superior maxillary bone, where it is continued in forming the zygomatic or maxillary spine.

In the ox the same crest ascends a good way towards the inferior margin of the orbit in a curved direction with the concavity inferior, to redescend afterwards on the external surface of the superior maxilla.

The ma.s.seter is an elevator of the lower jaw. It acts, above all, as in the human species, in the process of mastication.

=Temporal Muscle= (Fig. 89, 3).--The development of the temporal is in proportion to the energy of the movements of elevation which the lower jaw has to execute.

It arises from the temporal fossa, and is inserted into the coronoid process of the inferior maxilla.

Its development, enormous in the carnivora, is such that the muscle projects beyond its fossa. It is less voluminous in the horse, and still less so in the ox. In the latter, indeed, the temporal fossa, although deep, is of small extent (see Fig. 62, p. 119); the frontal bone being large, it is found to be thrown back on the lateral walls of the cranium, below the osseous processes which support the horns and overhang the fossa in question, as well as the muscle which it contains.

It is covered by the auricular muscles, and by the base of the pinna of the ear.

Like the ma.s.seter, the temporal is an elevator of the lower jaw.

Cutaneous Muscles of the Head

=Occipito-Frontalis.=--The epicranial aponeurosis is extremely thin. In the dog the occipital muscle occupies the superior part of the head; it overlies the temporal muscle.

With regard to the frontal muscle, which is of great extent in the ox (Fig. 91, F), it is represented in the horse and the carnivora by a small fleshy fasciculus only, the _fronto-palpebral muscle_, similar to the superciliary muscle. This, occupying the superior and internal part of the border of the orbit, ends by blending its fibres with those of the orbicular muscle of the eyelids at the region of the eyebrow.

=Orbicularis Palpebrarum= (Fig. 90, 2; Figs. 91, 92).--This annular muscle surrounds the palpebral orifice, and takes its origin on the internal part of the orbital region. In the horse it arises, by a small tendon, from a tubercle which occupies the external surface of the os unguis, or lachrymal bone.

This muscle determines the narrowing and closure of the palpebral orifice.

=Pyramidalis Nasi.=--The pyramidal muscle is not found in the domestic animals. It appears to be blended with the internal elevator of the upper lip and wing of the nose; this is easy of comprehension if we bear in mind the relative position of these two muscles in the human species.

[Ill.u.s.tration: FIG. 90.--MYOLOGY OF THE DOG: MUSCLES OF THE HEAD.

1, Ma.s.seter; 2, orbicularis palpebrarum; 3, zygomaticus major; 4, lachrymal (this muscle is sometimes described under the name of the small zygomatic); 5, levator labii superioris proprius; 6, levator labii superioris alaeque nasi; 7, caninus; 9, buccinator; 11, zygomatico-auricularis; 12, external temporo-auricularis; 14, parotid gland; 15, parotido-auricularis; 16, inferior maxillary bone; 17, digastric.]

=Corrugator Supercilii.=--This muscle is represented by the fronto-palpebral muscle noticed above, which is by some regarded as a vestige of the frontal.

=Zygomaticus Major= (Fig. 90, 3; Figs. 91, 92).--This is the _zygomatic-l.a.b.i.al_ of veterinarians. This muscle is of an elongated form, and has a ribbon-like aspect.

In the dog and the cat it arises from the base of the pinna of the ear, from the portion of this base which bears the name of scutiform cartilage. (With regard to this cartilage, see p. 242, =Zygomatico-auricularis=.) From this it is directed downwards and forwards, to terminate, after having crossed the ma.s.seter, on the deep surface of the skin of the corresponding l.a.b.i.al commissure.

This mode of termination is the same in the ox and the horse; but where the muscle differs is at the level of its upper extremity. There it ascends less than in the carnivora. In the ox it arises from the zygomatic arch in the neighbourhood of the temporo-maxillary articulation; in the pig and the horse its origin is still lower, on the surface of the ma.s.seter, close to the maxillary spine.

When it contracts, it draws upwards the l.a.b.i.al commissure.

Now, in man, we remember, it is the great zygomatic that, by an action of the same kind, determines the essential characters of the expression of laughing.

There is, accordingly, a connection to be established between those displacements which are similar and the a.n.a.logy of facial expression which necessarily results from them.[33]

[33] edouard Cuyer, 'The Mimic,' Paris, 1802.

=Zygomaticus Minor= (Fig. 90, 4; Figs. 91, 92).--The existence of this muscle has not been clearly demonstrated. Nevertheless, Straus-Durckheim noted its presence in the horse, and described it as 'a muscle arising by two heads, of which one, the superior, arises from the malar bone below the orbit, and pa.s.ses downwards and forwards over the fibro-adipose layer which supports the moustache. The second, the inferior, arises from the alveolar border in front of the second molar tooth, and pa.s.ses forward to be inserted into the same fibro-adipose layer.'[34]

[34] H. Straus-Durckheim, 'Anatomie descriptive et comparative du chat,' Paris, 1845, t. ii., p. 210.