Functional anatomy of the masticatory mechanism: A comparative study of physical characteristics of jaw- closing and jaw- opening muscles in sheep

The functional anatomy study of the mammalian muscles is very important for understanding the mastication mechanism during eating. It is also important for their health and breeding performance. Fifteen heads of sheep (Mean ± SD age: 18 ± 4 month) and of both sexes were included in this study. A comparative gross anatomical study and linear measurements of the two types of masticatory muscles were conducted on slaughtered sheep heads. The physiological crosssectional area (PCSA), maximal isometric force, torque and kinetic energy were calculated. were calculated. The study showed that the sheep jaw-closing muscles were the masseter and temporal muscles whereas the digastricus was a jawopening muscle. The physiological crosssectional area (PCSA) was 3.1, 2, 3.1 cm2 for the masseter, temporal and the digastric muscle, respectively. The maximal isometric force for those muscles was 6.2, 4, 6.2 cm2, respectively. The force of those muscles was 0.49, 0.098, 0.196 N, respectively. The torque of those muscles was 0.49, 0.098, 0.196 Nm, respectively. While the kinetic energy of the jaw-closing group was 90 Joule, that of the jaw-opening group was 30 Joule. The values of the PCSA, force, maximum isometric force, torque and kinetic energy of the jaw-closing group were higher than those of the jawopening. This is suggested to be due to the increased activity of the jaw-closing muscles. Keyword: Functional, anatomy, masticatory muscles, sheep Introduction The functional anatomical study of the masticatory muscles in sheep is very important to understand the mastication mechanism during eating. They are also important for the animal health and breeding success. There is very rare literature about the physical properties of the masticatory muscles that explain masticatory mechanism in sheep. The anatomy of the masticatory muscles was previously studied in ruminants and described carefully in details (Getty, 1976 and Dyce et al., 2010). Moreover; the function of the masticatory muscles correlates with resemblance generally skull shape and food natural of the deer reported by (Janis, 1990) . However, no detailed description of masticatory mechanics is available in sheep. The oro-facial system including stomatognathic system, maxillamandibular apparatus and masticatory system was a functional unit (Patial and Bindra, 2012), while the masticatory system was completed by including a jaw opener digastric muscle using the origin and insertion sites (Weijs et al., 1987) . The involvement actions of masticatory muscles in a variety of mammals in which feeding behavior and the configuration of the masticatory apparatus were different was reported by (Gans, 1978; Gorniak, 1985) . The mechanism of jaw opening and closing is complex (Herring, 1992; Hiiemae, 2000) . The mechanical advantage of the superficial layer of the masseter muscle was promoted by elevation of the jaw joint and expansion of the mandibular angle (Popowics and Herring, 2006) . The present work which aimed to investigate the functional anatomy of the different masticatory muscles in the sheep could be considered as supporting The present study was aimed to present the research concerning the comparison of the masticatory functional biomechanics of the closing and opening movements of the masticatory muscles in sheep in addition to understand the dynamics of masticatory muscles of adult’s sheep.