EXTRINSIC MUSCLES OF THE SKULL AND PECTORAL GIRDLE

The extrinsic muscles are those that connect the skull to either the axial or appendicular skeleton. These muscles are associated with the pectoral girdle and neck. Muscles from the pectoral girdle that link the skull to the appendicular skeleton include the m. Trapezius, the m. Episternocleidomastoid, and the m. Clavicle Dorsalis (Byerly 1925; Al-Hassawi 2004, 2007). All three muscles are innervated by branches of the second, third, and fourth spinal nerves (Byerly 1925). For information regarding other muscles of the pectoral girdle such as the m. Levator Scapulae, readers are directed towards Günther (1867), Osawa (1898), Byerly (1925), von Wettstein (1931), and Al-Hassawi (2004, 2007).

 m. Trapezius (MTrap)

The m. Trapezius is a wide sheet-like muscle that connects the shoulder girdle to the skull and vertebral column and serves to protract the forelimb (Byerly 1925). It originates from the fascia and fatty tissue along the crest of the vertebral column (Byerly 1925; Edgeworth 1935; Al-Hassawi 2004, 2007) but here we are most concerned with the anteriormost point of origin that is closest to the back of the skull. According to Byerly (1925) this comprises an origin from the parietal-squamosal arch superficial to the m. Depressor Mandibulae (Figure 21, Figure 22 and Figure 42). Similarly, Al-Hassawi (2004, 2007)) described the site of origin as the fatty tissue and muscle fascia on the posterodorsal margin of the skull but she does not consider the area of origin to include the midline (contra Byerly 1925). Tsuihiji (2007, p. 1011) describes the m. Trapezius (within a muscle group he called the m. Cucullaris Complex) as originating from the posterior surface of the parietal and squamosal between the m. Depressor Mandibulae and m. Longissimus Capitis, but he is probably referring to part of the m. Episternocleidomastoid or m. Clavicle Dorsalis rather than the m. Trapezius of other authors.

Al-Hassawi (2004, 2007, p. 105) describes the m. Trapezius as inserting on the anterior half of the dorsal edge of the clavicle. Byerly (1925, p. 24) instead describes this muscle as inserting on the dorsal third of the anterior border of the scapula (Figure 43).

m. Episternocleidomastoid (mEscm)

The m. Episternocleidomastoid serves to lift the scapula, but may also be used when moving the head. For this extrinsic muscle Byerly (1925) uses the term "cephalo-clavicularis" whereas Tsuihiji (2007) refers to it within the "m. Cucullaris complex."

The muscle originates from the anterior margins of the clavicle and interclavicle ventral to the insertion of the m. Trapezius (Figure 21, Figure 22, Figure 43, Figure 44, and Figure 45) (Byerly 1925; Al-Hassawi 2004, 2007; Tsuihiji 2007). Al-Hassawi (2004, 2007) described three separate insertions on the posterior surface of the skull:

- Branch 1 (mEscm1). Inserts on the posterolateral end of the paroccipital processes of the opisthotic (Figure 42).

- Branch 2 (mEscm2). Inserts above branch 1 on the posteromedial margin of the squamosal just above the contact with the paroccipital process of the opisthotic (Figure 42).

- Branch 3 (mEscm3). Inserts along the posterodorsal edge of the squamosal and parietal immediately ventral to the origin of the m. Depressor Mandibulae and dorsal to the m. Semispinalis Capitis. According to the figure provided by Al-Hassawi (2004, 2007, p. 117, figure 3.4a) the area of insertion for this branch is double the size of that for branches 1 and 2 (Figure 42).

As Tsuihiji (2007) points out, Fürbringer (1900) also reported that a portion of this muscle may insert on the quadratojugal.

m. Clavicle Dorsalis (mClDo)

The m. Clavicle Dorsalis originates from the anterolateral surface of the clavicle dorsal (Figure 21, Figure 22, Figure 43, and Figure 44) to the m. Episternocleidomastoid and it inserts onto the posterior surface of the parietal medial to the insertions of the m. Episternocleidomastoid (Figure 21, Figure 22, and Figure 42). It lies beneath m. Depressor Mandibulae and m. Trapezius (Al-Hassawi (2004, 2007). Byerly (1925) probably included this muscle as part of the "cephalo-clavicularis" whereas Tsuihiji (2007) included it within the "cucullaris complex."