Artistic Anatomy of Animals Part 17

It is inserted, in the case of the dog and cat, into the mastoid process, where it is united with the mastoido-humeral; in the ox it is divided into two portions--one which goes to the base of the occipital bone, the other pa.s.sing in front of the ma.s.seter is by the medium of the aponeurosis of this latter attached to the zygomatic crest. This latter part is considered by some writers as forming a portion of the panniculus muscle of the neck.

In the horse it is attached to the angle of the lower jaw by a tendon, which an aponeurosis that pa.s.ses under the parotid gland binds to the mastoido-humeral muscle and the mastoid process.

By reason of this insertion into the jaw, in the case of the solipeds, this muscle is further named the _sterno-maxillary_.

When it contracts, it flexes the head, and inclines it laterally. This movement is changed to direct flexion when the two sterno-mastoid muscles contract simultaneously.

In man, the sterno-cleido-mastoid and the trapezius leave a triangular s.p.a.ce between them, which, being limited inferiorly by the middle third of the clavicle, is known as the supraclavicular region; this region, being depressed, especially in its inferior part, has also been given the name of supraclavicular fossa--popularly called the '_salt-cellar_.'

The muscles which form the floor of this region, pa.s.sing from above downwards, are: a very small portion of the complexus, splenius, levator anguli scapulae, posterior scalenus, and anterior scalenus; then, crossing these latter, and most superficial, is the omo-hyoid muscle.

An a.n.a.logous region, but of only slight depth, exists in quadrupeds; its borders are formed by the mastoido-humeral and trapezius muscles.

It is not limited below by the clavicle--we know, indeed, that this, or the intersection which represents it, belongs to the mastoido-humeral muscle--but by the inferior portion of the spine of the scapula.

It is of greater or less extent according to the species considered.

In the dog, cat, pig, and ox, it is narrow, for the muscles which bound it approach one another pretty closely. It has, as in man, the form of a triangle, with the apex above. In the horse it is much broader, and, contrary to the arrangement which it presents in the human species, the widest part is directed upwards.

The muscles which we find there are, consequently, more or less numerous. In the dog and cat they are: a portion of a muscle which we do not normally meet with in man--the _omo-trachelian_--then in a decreasing extent: supraspinatus, levator anguli scapulae and splenius.

In the pig: the omo-trachelian, supraspinatus, and the terminal portion of the sterno-prescapular--the anterior part of the lesser or deep pectoral muscle.

In the ox: the omo-trachelian only.

But in the horse we find the omo-trachelian, the supraspinatus, and the terminal extremity of the sterno-prescapular; then in a larger extent of area the levator anguli scapulae and the splenius; and, finally, the anterior portion of the rhomboid.

Among the muscles which we have just enumerated are some that we have already studied; these are the sterno-prescapular and the rhomboid. We will examine the supraspinatus muscle in connection with the region of the shoulder.

As to the scaleni muscles and the complexus, they are deeply situated, whereas the omo-hyoid is visible in the anterior region of the neck only.

There remain for us, accordingly, to examine, at the present juncture, but the omo-trachelian, levator anguli scapulae, and splenius muscles.

=The Omo-trachelian Muscle= (Fig. 68, 13; Fig. 69, 17; Fig. 70, 20).--Also called the _acromio-trachelian_, _levator ventri scapulae_,[21] the _angulo-ventral muscle_, and the _transverso-scapular_,[22] etc., this muscle is described by some hippotomists as belonging to the mastoido-humeral, of which it then forms its posterior or deep portion (see p. 153).

[21] Ventri, because inserted into the inferior part of the spine of the scapula, towards the acromion--that is, on the ventral side--by contrast with the trapezius, which is attached higher up (dorsal side) on the same process.

[22] Among the many names given to this muscle, Arloing and Lesbre recommend the adoption of the name 'transverse scapular' given by Straus-Durckheim, or 'transverse of the shoulder' (Arloing and Lesbre, 'Suggestions for the Reform of Veterinarian Muscular Nomenclature,' Lyons, 1898).

The omo-trachelian muscle is found in all mammalia, man alone excepted.

It is, however, sometimes found in the human being; but it then const.i.tutes an anomaly.

In the dog, pig, and ox, it arises from the inferior part of the spine of the scapula, in the region of the acromion, and terminates on the lateral portion of the atlas.

In the cat it is attached besides to the base of the occipital bone. It is visible in the s.p.a.ce limited by the trapezius and the mastoido-humeral, the direction of which it crosses obliquely.

In the horse it appears to be blended in clearly defined fashion with the mastoido-humeral. Attached below, like this latter, to the anterior border of the humerus, it covers the scapulo-humeral angle; and is attached by its upper portion to the transverse processes of the first four cervical vertebrae.

We remember that the transverse processes are often, from their relation with the trachea, known as the tracheal processes. Hence the word 'trachelian,' which forms part of the name of the muscle with which we are now dealing.

By its contraction it helps to draw the anterior limb forwards.

When this muscle, as an abnormality, exists in man, it arises from the clavicle or the acromion process, traverses the supraclavicular fossa, and is inserted into the transverse processes of the atlas or axis, or of both these vertebrae, or of the cervical vertebrae below these latter. It is then known by the names of the _elevator of the clavicle_ or _elevator of the scapula_, and, finally, as the _cleido-omo-transversalis_ (Testut).[23]

[23] L. Testut, 'Les anomalies musculaires chez l'homme expliquees par l'anatomie comparee,' Paris, 1884, p. 97. A. F. Le Double, 'Traite des variations du systeme musculaire de l'homme et de leur signification au point de vue de l'anthropologie zoologique,' Paris, 1897, t. i., p. 235.

=The Levator Anguli Scapulae= (Fig. 68, 15; Fig. 70, 23).--As we have pointed out (p. 136), the levator anguli scapulae, because of its connections with the great serratus, is sometimes described with it. But inasmuch as in human anatomy these two muscles are considered separately, and that, in the superficial layer of muscles, they are seen in different regions--the great serratus in the thoracic, and the levator anguli scapulae in the cervical--we prefer to study them separately.

We remember that in man this muscle arises from the transverse processes of the upper cervical vertebrae and is inserted into the superior portion of the spinal border of the scapula, into the portion of this border which is situated above the spine; it also contributes to the formation of the floor of the supraclavicular region.

When it contracts, it draws the superior portion of the scapula forwards and upwards, and causes a see-saw movement, for at the same time the inferior angle of the scapula is directed backwards. Taking its fixed point at the shoulder, it directly extends the neck if the muscle of one side acts at the same time as that of the opposite; but if only one muscle contracts it inclines the neck to the corresponding side.

It is to be noticed that during movements a little more active than the ordinary the levator anguli scapulae, as moreover the other muscles of the neck do, becomes very distinct. We have, indeed, often remarked that, apart from these movements, each time the support of one of the fore-limbs is brought into requisition a brusque contraction of the muscles of this region accompanies it.

This contraction gives the impression that, as on the one hand, each support determines a momentary arrest of progression, a jolt, and on the other hand, the head continues to be projected in the forward direction, the latter should be retained. But it cannot be so except by an effort in the opposite direction--that is to say, by the brusque contraction which we have just pointed out.

a.n.a.logous contractions also take place in a man while running at the beginning of each contact of the lower limbs with the ground.

We may add, apropos of this latter, that displacements of the head, sometimes in very p.r.o.nounced fashion, take place during simple walking, and that every time one of the lower limbs is carried forwards the head is projected in the same direction. These displacements, which we also find take place in the horse in pacing, especially in the region of the neck and head, seem then to have the effect of aiding the progression of the body forwards.

They occur especially in animals when drawing a heavy load, and in individuals whose walking movements are executed with difficulty.

It is necessary to repeat that, in these cases, the individual appears to a.s.sist the movement of his body by the impetus which the projection of his head forward determines, in order to add--and it is for this that we have referred to the subject--that during the intervals between each projection the head is carried backwards by a muscular contraction similar to that above discussed.

=The Splenius= (Fig. 68, 14; Fig. 70, 22).--In man, this muscle is attached in the median line to the inferior half or two-thirds of the posterior cervical ligament, to the spinous processes of the seventh cervical, and four or five upper dorsal vertebrae; it pa.s.ses obliquely upwards and outwards, becomes visible in the supraclavicular region, pa.s.ses under the sterno-cleido-mastoid, and proceeds to duplicate the cranial insertions of this latter; and, further, the most external fasciculi of this muscle are inserted into the transverse processes of the atlas and the axis.

These separate superior attachments, and the division of the muscle which results, have caused the splenius to be regarded as formed of two portions: splenius of the head, and splenius of the neck.

In the horse, this muscle, which is of voluminous dimensions, arises from the superior cervical ligament, and the spinous processes of the first four or five dorsal vertebrae; thence it proceeds to be inserted into the mastoid crest, and the transverse processes of the atlas and three or four vertebrae following.

The region occupied superficially by the splenius is remarkable for the prominence which this muscle, with the deeply-seated complexus, which is equally bulky, determines at this level; it is situated above that region of the neck, in which are seen in part the fasciculi of the levator anguli scapulae. It terminates above and in front in the ridge, which is sometimes very p.r.o.nounced, which the transverse processes of the atlas make on each side of this part of the neck.

In the dog and the cat, the superior and anterior region of the neck is thick and of rounded form, on account of the development which the splenius presents in those animals; but it is covered by the mastoido-humeral.

This latter relation is also found in the ox, but the splenius in this case is but slightly developed.

When the splenius contracts it extends the head and neck, while inclining them to its own side.

If the splenius of one side contracts at the same time as that of the opposite, the extension takes place in a direct manner--that is to say, without any modifying lateral movement.

Infrahyoid Muscles

Having studied the lateral surfaces of the neck, we must now examine the anterior part of this region. Here, between the two sterno-mastoid muscles, we find a s.p.a.ce broader above than below, in which are situated the larynx and the trachea, to the general arrangement of which is due the cylindrical form which this region presents. This s.p.a.ce corresponds to that which in the neck of man is limited laterally by the sterno-cleido-mastoid muscles, below by the fourchette of the sternum, and above by the hyoid bone. In animals, as in man, it is called the infrahyoid region.

The hyoid bone in quadrupeds is situated between the two rami or branches of the lower jaw. Owing to this disposition, the region above this bone, instead of having its surface projecting a little beyond the inferior border of the maxillary bone, is depressed. This is especially so in the horse. It is there that we find in this animal the region known as the _trough_ (_auge_); the larynx corresponds to that part known as the _gullet_.

The muscles which occupy the infrahyoid region are: the sterno-thyroid, the sterno-hyoid, and the omo-hyoid. There is also a thyro-hyoid, but because of its deep situation and its slight importance it offers no interest from our point of view.