The dense concentration in cardiac muscle of orderly arrangements of interdigitating actin and myosin molecules, which could synchronously slide across each other throughout the atrial or ventricular muscle, resulted in an organ that could make strong, quick contractions of short duration. And so, between the third and fourth week, the cardiac muscle of the single-tube heart begins to contract. The bundles, nodes, and Purkinje fibers, which are the components of the conducting system of the heart, are merely modified cardiac muscle fibers.

If damaged, smooth muscle is able to regenerate to a limited degree by division of preexisting muscle cells and by division and differentiation of nearby connective tissue cells of the mesenchymal type. However, there is no regeneration of damaged cardiac muscle; repair of damaged myocardium is by means of fibrous scar tissue.

SKELETAL MUSCLE

Skeletal muscle is also known as voluntary, striated, striped, or segmental muscle. The last term refers to the origin of most of the skeletal muscles of the vertebrate body from the segmented paraxial mesoderm, the somites.

In the adult prevertebrate amphioxus, there are, according to the species, from 50 to 85 muscle segments known as myotomes, or myomeres (see Plate 1-1). The V-shaped myotomes are dovetailed into one another along the length of the body. The individual striated muscle fibers of each myotome run parallel to the long axis of the body, and each myotome receives a pair of nerves from the dorsal nerve cord. The original myotomic segments are retained in a similar fashion throughout the trunk of adult fish. However, each myotome is divided into a dorsal, or epaxial, and a ventral, or hypaxial, portion, which are separated in fish by the transverse processes of the vertebral column and a fibrous septum extending from these processes to the lateral body line. Each myotome is supplied by a spinal nerve, with a dorsal ramus innervating the epaxial portion and a ventral ramus innervating the hypaxial portion.

In the human embryo, the maximum number of 42 to 44 somites is attained during the fifth week, after which the first of the four occipital and the last seven or eight coccygeal somites regress and disappear. In addition to the somites, there are three masses of mesenchyme on each side of the embryonic head that are anterior to the otic vesicles—the future membranous labyrinths of the inner ears—which represent the three pairs of preotic somites found in primitive vertebrate embryos that give rise to the striated extrinsic muscles of the eye. The three preotic mesenchymal masses in the human embryo aggregate into one mass around the developing eyeball during the fifth week, giving rise to the extrinsic ocular muscles that become innervated by the initially nearby oculomotor (III), trochlear (IV), and abducens (VI) nerves (see Plate 1-16).

In the human embryo, the early differentiation of all the persisting somites (the second occipital to the third or fourth coccygeal) is similar: the ventromedial portion of the somite becomes the sclerotome; the sclerotomal cells migrate toward the notochord to give rise to the vertebral column and ribs, and the remaining portion of the somite is then called the dermomyotome (a fluidfilled cavity, the myocoele, appears in the somite but is soon obliterated); the cells of the dermomyotome then proliferate to form a medial mass, the myotome, which can be distinguished from the less proliferative lateral portion, the dermatome (see Plates 1-2 and 1-3). Finally, the cells of the dermatome spread beneath the overlying ectoderm to give rise to the subcutaneous fascia and the dermis of the skin. The segmental dermatome distribution of the embryo is reflected in the innervation of the skin of the trunk and limbs of the adult. The area of skin supplied by a single spinal nerve in the adult constitutes a dermatome.