The hypaxial origin of the epaxially located rhomboid muscles.

In vertebrates, skeletal muscles of the body are made up of epaxial and hypaxial muscles based on their innervation and relative position to the vertebral column. The epaxial muscles are innervated by the dorsal branches of the spinal nerves and comprise the intrinsic (deep) back muscles, while the hypaxial muscles are innervated by the ventral branches of the spinal nerves including the plexus and consist of a heterogeneous group of intercostal, abdominal, and limb as well as girdle muscles. The canonical view holds that the epaxial muscles are derived from the medial halves of the somites, whereas the hypaxial muscles are all derived from the lateral somitic halves. The rhomboid muscles are situated dorsal to the vertebral column and therefore in the domain typically occupied by epaxial muscles. However, they are innervated by a ventral branch of the brachial plexus called the N. dorsalis scapulae. Due to the apparent inappropriate position of the muscle in relation to its innervation we investigated its origin to help clarify this issue. To study the embryonic origin of the rhomboid muscles, we followed derivatives of the medial and lateral somite halves using quail-chick chimeras. Our results showed that the rhomboid muscles are made up of cells derived mainly from the lateral portion of the somite. Therefore the rhomboid muscles which lie within the epaxial domain of the body, originate from the hypaxial domain of the somites. However their connective tissue is derived from both medial and lateral somites.

Internal jugular vein cannulation complications and elimination of the muscular triangle of the neck due to aberrant infrahyoid muscles.

We report on a rare case of anatomical variations of the infrahyoid muscles with prominent clinical significance. The aberrant anatomy was on the right side of the neck and involved the omohyoid and sternohyoid muscles. The superior belly of the omohyoid was duplicated in width due to an aberrant belly anteriorly and merged with fibers of the inferior belly inferiorly and the sternohyoid muscle medially. An additional aberrant muscle slip extended between the inferior third of the sternohyoid muscle and united with the inferior belly of the omohyoid. The intermediate tendon between the two bellies of the omohyoid was absent, whereas the so-called muscular triangle of the neck was diminished. Due to the arrangement and fusion of myofibers the muscle could be termed as omo-sternohyoid muscle. A profound hematoma was noted in the aberrant muscle at the area overlying the internal jugular vein indicating difficulty in obtaining jugular venous access for catheter placement. Clinicians and surgeons should be aware of muscular anatomic variations when intervening in the lateral neck area as the classical anatomical landmarks might be misinterpreted and confuse.