All of the following are Digastric muscles except Sternocleidomastoid

Question 5

All of the following are Digastric muscles except

a.       Muscle fibres in the ligament of Treitz

b.      Omohyoid

c.       Occipitofrontalis

d.      Sternocleidomastoid

Answer

D) Sternocleidomastoid

Reference

Gray 38^th Edition Pages 805, 806, 807, 1763, 1764

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Discussion

The digastricus (digastric muscle) consists of two fleshy bellies united by an intermediate rounded tendon. The two bellies of the digastric muscle may have different embryological origins, and are supplied by different nerves.

Explanations

a)      The terminal part and the duodenojejunal flexure are said to be positioned by the 'suspensory muscle of the duodenum' (suspensory muscle, or ligament, of Treitz), often described as being in two parts:

Ä     a slip of skeletal muscle derived from the diaphragm near its oesophageal opening, ending in connective tissue near the coeliac artery

Ä     a fibromuscular band of smooth muscle, passing from the  duodenum (third and fourth partsand duodenojejunal flexure) to blend with the same pericoeliac connective tissue.

b)      Omohyoid consists of two bellies united at an angle by an intermediate tendon. Superior belly: branches from the ramus superior of the ansa cervicalis (C1). Inferior belly: the ansa cervicalis itself (C2, 3).

c)      Occipitofrontalis covers the dome of the skull from the highest nuchal lines to the eyebrows. The occipital part is supplied by the posterior auricular branch, and the frontal part by the temporal branches of the facial nerve.

d)      Sternocleidomastoid (7.63) descends obliquely across the side of the neck and forms a prominent surface landmark (7.62), especially when contracted. It is thick and narrow centrally, and broader and  thinner at each end.

Comments

Treítz  described both entities, naming the former der Hilfsmuskel (the accessory muscle). Subsequent authorities  regarded them as a digastric muscle, naming the whole the suspensory muscle of Treitz, a misnomer perpetuated in most textbooks. Confusion was increased by Haley & Peden, who derived the 'suspensory muscle' from the right crus, and by Argème et al, who described an intermediate tendon but regarded this as part of a 'false' digastric muscle. Jit has persistently repeated the dual nature of the original description by Treítz, supporting it by embryological and histological evidence. The diaphragmatic slip (Hilfsmuskel) has no satisfactory official name.

Tips

It is supplied, according to Jit, by myelinated nerve fibres probably from the phrenic nerve and is sometimes considered an aberrant part of iliocostalis thoracis. The suspensory muscle proper (smooth muscle) is supplied by autonomic fibres from the coeliac and superior mesenteric plexuses. Descriptions of the duodenal attachments of the muscle vary; none of these accounts contain a convincing view of its function, the usual suggestion being that it augments duodenojejunal flexure, acting like a valve.Muscle fibres in the ligament of Treitz