Journal of Histochemistry & Cytochemistry 60(9) 638 –657 © The Author(s) 2012 Reprints and permission: sagepub.com/journalsPermissions.nav

The thoracic diaphragm (diaphragm, as commonly referred to) is a complex skeletal muscle organ essential for support-ing respiration in mammals (De Troyer and Estenne 1988; Rowley et al. 2005). The diaphragm also exerts nonrespira-tory functions, including promoting venous return, lymphatic sieving, and gastroesophageal reflux restriction (De Troyer et al. 1981; Martin et al. 1983; Mittal 1993; Abu-Hijleh et al. 1995; Loukas et al. 2005). This diaphragm multitasking is supported by a unique anatomy. The diaphragm is divided into costal and crural muscle domains with an interceding central tendinous domain (Blair 1923; Anraku and Shargall 2009). The costal (ventral) domain is the larger component of the diaphragm and is arrayed radially from the central tendon to the ribs. The crural (posterior) domain tacks the diaphragm to the dorsal wall and the spinal column, and it wraps and provides apertures for the esophagus and the aorta. The cen-tral tendon, primarily a supportive connective tissue struc-ture, provides passage of the inferior vena cava. Whether the costal and crural domains represent two dis-tinct muscles, possibly of different embryological origins, has been a subject of debate (Pickering and Jones 2002; Babiuk et al. 2003). While the crural and costal muscles act in synchrony throughout respiration, the activities of these two muscular regions can diverge during certain events, such as swallowing and emesis (Pickering and Jones 2002). 452417 JHCXXX10.1369/0022155412452417Stuels atz et al.Defining the Diaphragm Muscle and Tendon Domain