The middle scalene muscle originates from middle portion of the first rib to the transverse processes of the 6th to 3rd vertebrae. It is located dorsally from the ventral scalene muscle. In many species, it is separated with ventral scalene by the brachial plexus, but this boundary is not present in carnivores like dogs, causing a lot of problem of definition...
Origin: middle portion of first rib
Insertion: transverse processes of C3-C6 (Su also C1 and C2)
Action: Draws the neck downward. In unilateral action, to bend the neck sideward. When the neck is fixed, the supracostal part can act in inspiration.
Nerve: ventral branches of the cervical and thoracic nerves.