The first description of the double-bellied condition of the digastric muscle in the finless porpoise Neophocaena phocaenoides and Dall's porpoise Phocoenoides dalli

In Humans, the digastric muscle, one of the suprahyoid muscles, consists of two parts, or bellies, which are connected by an intermediate tendon, which in turn is connected to the body of the hyoid bone by a loop of fibrous connective tissue. The anterior belly of the digastric muscle is supplied by the trigeminal nerve, and the posterior belly by a facial nerve (Woodburne and Burkel 1994). The form of the digastric muscle has been described in so many mammals (Dobson 1879; Bijvoet 1908; Toldt 1908; Getty 1975; Tomo et al. 1998), that Bijvoet (1908) classified dozens of mammalian species into three groups based on the anatomy of their digastric muscles. These three groups are: 1) mammals with no digstric, 2) mammals whose digastric has two bellies united by an intermediate tendon, and 3) mammals with a digastric muscle lacking an intermediate tendon between the two bellies. All examples with double bellies are innervated by both facial and trigeminal nerves. According to Getty (1975), horses have a double-bellied digastric muscle, which arises from the jugular process of the occipital bone and attaches to the mandible, while the singlebellied condition is reported in pigs and goats, in which the posterior bellies are considered to be degenerated. Dogs have a single-bellied muscle that runs from the paracondylar process of the occiput to the ventral border of the mandible. But Evans (1993) stated that “although it appears as a single-bellied muscle in the dog, a tendinous intersection and innervation by both the n. trigeminus and the n. facialis are evidence of its dual nature. Thus, the two parts of the muscle are referred to as the rostral belly and the caudal belly.” Various old descriptions seem to refer to a doublebellied condition among the odontoceti. The M. occipitohyoideus of the harbor porpoise Phocoena phocoena described by Rapp (1837) and Stannius (1849) can be interpreted, based on their origin and insertion, as the homologue of the posterior belly of the digastric muscle. Schulte and Smith (1918) described a “hyomandibularis” and a “depressor mandibulae” in the pygmy sperm whale Kogia breviceps on the basis of the innervation of those two muscles, which makes them equivalent to the anterior and posterior bellies of the digastric. Subsequently, however, the digastric has been described as a singlebellied muscle in many odontocetes, such as the narwhal Monodon monoceros (Howell 1930), striped dolphin Stenella coeruleoalba (Hosokawa and Kamiya 1965; Lawrence and Schevill 1965; Werth 1992; Reidenberg and Laitman 1994), Atlantic spotted dolphin S. frontalis (Lawrence and Schevill 1965), Pacific white-sided dolphin Lagenorhynchus obliquidens (Reidenberg and Laitman 1994), Atlantic white-sided dolphin L. acutus (Lawrence and Schevill 1965; Werth 1992; Reidenberg and Laitman 1994), whitebeaked dolphin L. albirostris (Lawrence and Schevill 1965; Werth 1992), common dolphin Delphinus delphis (Lawrence and Schevill 1965; Pilleri 1976; Werth 1992; Reidenberg and Laitman 1994), bottlenose dolphin Tursiops truncatus (Lawrence and Schevill 1965; Werth 1992; Reidenberg and Laitman 1994), Risso’s dolphin Grampus griseus, long-finned pilot whale Globicephala melas (Werth 1992; Reidenberg and Laitman 1994), killer whale Orcinus orca (Werth 1992), True’s beaked whale Mesoplodon mirus, Gervais’ beaked whale M. europaeus (Reidenberg and Laitman 1994), sperm whale Physeter macrocephalus (Werth 1992; Reidenberg and Laitman 1994), dwarf sperm whale K. sima (Reidenberg and Laitman 1994), and even in the finless porpoise Neophocaena phocaenoides