The auxiliary respiratory muscles include the scalene, the sternocleidomastoid, the pectoralis major, the trapezius, and the external intercostals. The scalene muscles are located in the neck, and they can be divided into three muscles:
- Anterior scalene: this muscle lies deep in the neck behind the sternocleidomastoid muscle. It originates from the C3-C6 vertebrae and attaches to the inner border of the first rib.
- Middle scalene: originates from C2-C7, and it’s the largest and longest of the three scalenes. It attaches to the upper surface of the first rib.
- Posterior scalene: this is the smallest of the three scalene muscles. It originates from C5-C8 and inserts into the outer surface of the second rib.
These muscles are able to elevate the first and second ribs. That is why they can also act as accessory muscles of respiration, along with the sternocleidomastoid muscle. The sternocleidomastoid muscle plays a role in the rotation of the head and flexion of the neck. This muscle originates from the manubrium of the sternum and clavicle, and attaches to the mastoid process of the temporal bone.