Visceral Muscles

musculature of internal organs in man, vertebrates, and acraniate animals. The visceral muscles include the muscles of the skin and skin glands, blood-vessel walls, excretory ducts of the urogenital system, intestines, pharynx, and heart. Visceral musculature is principally smooth; in the heart and pharynx it is transversely striated. It is formed of lateral plates (some muscles are of ectoderm or dermatome) and is innervated by the visceral nerves. In the pharynx of lower fishes and cyclostomes, which is pierced by visceral fissures, the visceral muscles are formed by the common constrictor, which in the region of the jaws is innervated by the trigeminal nerve; in the region of the hyoid arch, by the facial nerve; in the region of the first gill slit, by the glossopharyngeal nerve; and in the remaining gill arches, by branches of the vagus nerve. Separate muscles that govern the movements of the visceral arches become detached from the common constrictor; these muscles acquire major importance in higher fishes, in which the common constrictor atrophies owing to the development of the gill cover. In the jaws these muscles are the mandibular adductor, the levator of the palatoquadrate cartilage, and the intermandibular. In fishes the trapezius muscle is separated from the posterior portion of the common constrictor and goes to the shoulder girdle of the fins. In terrestrial vertebrates with development of the autostyl the levator of the palatoquadrate cartilage atrophies. (It has been preserved in modified form in some reptiles and birds.) The muscle that adducts the mandible becomes divided into the masseter, temporal, and pterygoid. From the muscles of the hyoid arch have arisen the depressor of the lower jaw and the subcutaneous musculature of the neck and face, which in man and apes is a part of the muscles of facial expression. With the atrophy of the branchial arches in terrestrial vertebrates, those muscles were converted into muscles of the hyoid apparatus, the pharynx, and the larynx. The trapezius muscle lost its relationship to the visceral apparatus and was converted into a muscle of the shoulder girdle.

N. S. LEBEDKINA