Trunk and Head Musculature in Fish Anatomy
Trunk Musculature
Trunk musculature in fish consists of a series of blocks of muscle called myotomes or myomeres, separated by connective tissue myosepta. These are representative of the segmentation of the trunk. Myotomes are composed of red and white muscle cells (myofibers). Red fibers are oriented at a 45° angle, roughly parallel to the body, while white fibers form the bulk of the myotomes, lying flat just under the skin, with their outer edges extending from side to side.
Parietal muscles lying above the horizontal septum are called epiaxial muscles, while parietal muscles lying below the horizontal septum are called hypoaxial muscles. Epiaxial muscles consist of two or more longitudinal dorsal bundles on each side, extending from the head to the base of the tail. Hypoaxial muscles consist of one longitudinal lateral bundle, extending from the pectoral girdle to the tail, and two longitudinal ventral bundles, extending from the pectoral girdle to the pelvic girdle. A linea alba, known as the ventral midline, separates the left and right hypoaxial muscles.
Cephalic Musculature
Cephalic musculature consists of the muscles that move the branchial arches and jaws, which can be divided into three groups: superficial constrictors, levators, and intracranial muscles.
Superficial Constrictors
The superficial constrictors cover the head and branchial region, from the eye to the branchial openings. They consist of dorsal and ventral constrictors that meet at the branchial openings. Superficial constrictors are numbered 1 to 6, with each constrictor corresponding to the same numbered branchial arch. The pharyngeal constrictors compress the chamber, expel water, and close the branchial openings. The second constrictor, dorsal to the hyoid arch and jaw, and ventral to the mouth opening, assists in the mouth. Constrictors 3 to 6 are similar, with connective tissue attached to each other, overlapping partially with the constrictor in front.
Constrictor of the Second Arch
The second constrictor, corresponding to the second hyoid arch, runs from the jaw angle and continues anteriorly to the second branchial opening and the EPII, just in front of the spiracle. It originates from the otic capsule and fascia surrounding it, and inserts on the omandibular cartilage. Intermandibular muscles lie on the ventral midline posterior to the jaw, just below the intermandibular cartilage. The mediоventral area causes the insertion on the ceratohyal. Like the other constrictors, EPII and the intermandibular muscles compress the branchial bags.
Levators
The levator group consists of three muscles: the levator maxillae superioris, the levator arcus palatini, and the levator hyoidei. The levator maxillae superioris represents the levator of the first branchial arch, lying anterior to the cranio-maxillary. It arises from the otic capsule and inserts on the palatoquadrate next to the quadratomandibular, acting as an upper jaw elevator. The second levator, the levator hyoidei, lies deep to EPII, and the two muscles may merge.
Cucullaris represents the levator of branchial arches 3 to 7. It is a large triangular muscle lying dorsal to the constrictors, extending from the waist to the pectoral girdle. It originates from the dorsal longitudinal bundles and inserts on the cartilage of the epibranchial arch and the scapular process of the pectoral girdle.
Intrinsic Muscles
Intrinsic muscles are small muscles that act on the cartilage of the branchial arches to expand the pharynx. They consist of two sets: medial intrinsic muscles, extending from the pharyngobranchial to adjacent cartilage, and lateral intrinsic muscles, extending from the pharyngobranchial to the epibranchial. The corresponding muscles are the common hypobranchial coracoarcualis, coracomandibularis, coracohyoideus, and coracobranchiales.
The coracobranchiales lie within the deep branchial bags. The coracobranchiales consist of five parts, which insert on the branchial arches 2 to 5 and the basibranchial cartilage. They pass the afferent branchial arteries and veins. The fifth coracobranchial and the coracoarcualis together form the anterolateral wall of the pericardial cavity.