Trapezius

M. trapezius

Thin triangular muscle sheet joining the dorsal neck and withers to the scapular spine.

Draft, not yet reviewed by a veterinarian. Content may contain errors. It is published so the community can review it.

This piece is a schematic shape, not a scan: there is no freely licensed model for this structure yet.

3D model of this piece: Vet Atlas procedural musculature, CC BY-SA 4.0

Overview

It is the most superficial muscle of the scapular region. It has two parts, separated by an aponeurotic strip over the scapular spine:

Cervical part (pars cervicalis): from the nuchal ligament to the scapular spine. Its fibres run caudoventrally.
Thoracic part (pars thoracica): from the spinous processes of the cranial thoracic vertebrae, which form the withers, to the scapular spine. Its fibres run cranioventrally.

Funicular part of the nuchal ligament (cervical part) and supraspinous ligament of the cranial thoracic vertebrae (thoracic part).

Tuber of the scapular spine (tuber spinae scapulae).

Raises and abducts the thoracic limb. The cervical part draws the scapula craniodorsally and the thoracic part caudodorsally.

Dorsal branch of the accessory nerve (n. accessorius, cranial nerve XI).

The thoracic part lies under the front of the saddle. A saddle that bears on the withers or pinches the scapular region causes pain and, over time, atrophy of this muscle, seen as a hollow behind the scapula.
Deep to it, over the spinous processes of the withers, lies the supraspinous bursa. Its infection ("fistulous withers") has been linked to Brucella abortus and Actinomyces bovis, and causes a swelling that ends up draining through sinus tracts.