الفهرس 
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Abstract
In the present study, we demonstrated the gross morphological differences and the arterial supply of the masticatory muscles in sheep and dogs with special reference to their spatial relationship to the temporo-mandibular joint and its effect on the feeding behavior in the two animal species.
In sheep, the largest muscle was the masseter which was broad, flat, and had principal role in the grinding movement. It was clearly divided into nine layers according to the direction of muscle fibers and the tendinous sheet of each layer. These layers were identified as follow: 1st superficial, 2nd superficial, intermediate, anterior deep, first posterior deep, second posterior deep, 1st maxillo-mandibular, 2nd maxillo-mandibular and zygomatic-mandibular layers. Meanwhile in dogs, the masseter muscle was relatively smaller in size and composed of eight laminar portions similar to those in sheep except the 2nd maxillo-mandibular layer which was absent.
The temporal muscle in sheep was small in size, while it was relatively larger in dogs to help the scissor like movement of the joint. It was more muscular in dogs and separated into superficial and deep layers by tendon.
We observed that the medial pterygoid muscle was separated in sheep into four layers according to the anatomical features of the tendinous sheet. These layers are identified from medial to lateral as: the 1st, 2nd, 3rd and the 4th Layers. The alternative pattern of origin and insertion of the four layers with tendinous and muscular portions increase the strength of the muscle. Meanwhile, it was separated in dogs into only three layers where the 4th layer was absent.
The lateral pterygoid muscle was the smallest muscle in this group and completely covered by the medial pterygoid muscle in both sheep and dogs. The digastric muscle was generally originated from the jugular process of the occipital bone and inserted into the medial surface of the body of the mandible in sheep and dogs. In sheep, the digastric muscle had two bellies with a short and thick intermediate tendon; the bellies were round and thick. In dogs, it was relatively more muscular than that of sheep, and had two bellies separated by a short intermediate tendon.
In the present study, we also investigated the arterial supply of the masticatory muscles in sheep and dogs. The masseter muscle in sheep was supplied by branches derived from the external carotid, transverse facial, lingual, superficial temporal, buccal, deep temporal and inferior labial arteries. Meanwhile in dog, it was originated from branches of the external carotid artery or its terminal divisions, facial, masseteric branch of superficial temporal, transverse facial, masseteric branch of caudal deep temporal, and the buccal arteries.
The temporal muscle in sheep received its arterial supply from branches of rostral and caudal auricular, superficial and deep temporal and the buccal arteries, while in dogs there were large numbers of branches such as the caudal auricular, superficial temporal, rostral and caudal deep temporal and buccal arteries. The medial pterygoid muscle in sheep received its arterial supply from branches of external carotid, lingual, caudal branch of rostral epidural rete mirabile, mylohyoid branch of inferior alveolar and buccal arteries. On the other hand, the medial pterygoid muscle in dog took its arterial supply through branches of the facial artery, mylohyoid branch of caudal deep temporal, rete branch and pterygoid branch of maxillary artery.
The lateral pterygoid muscle had its arterial supply in sheep from pterygoid branch of the caudal branch of rostral epidural rete mirabile and the buccal artery in addition to maxillary artery, which had a minor role in its arterial supply. Meanwhile, the lateral pterygoid muscle in dog was supplied only by small branches from mylohyoid branch and pterygoid branch of the maxillary artery.
The digastric muscle had caudal and rostral bellies, each of which had a separate pattern in its arterial supply variable in the two animals. The caudal belly of digastric muscle in sheep received branches from the caudal auricular artery while the rostral belly received branches from lingual and sublingual arteries. The caudal belly in dog received a branch from the caudal auricular artery and external carotid artery, while the rostral belly received blood from branches originated from the facial and sublingual arteries.
To identify the functional roles of the masticatory muscles in sheep and dog, we investigated the biometrical features of each of the masticatory muscles for further assessment of the jaw movements and subsequent relation with the feeding habits. We traced the comprehensive formation of the masticatory muscles with special reference to their spatial relationships to the TMJ.
In the current study we found that the main feature of sheep head was bony in compared with dogs head which was highly muscular. The ratio between the weights of masticatory muscles in sheep to the weight of the mandible was (1:1) while the ratio in dog was (3:1). The masseter muscle was the greatest component of masticatory muscles in sheep meanwhile the temporal was comparatively the main muscle in dog.
In sheep, the position of the TMJ was also suited for great mobility in the transverse plane but with limited opening. These configurations enable more developed side to side movement during chewing, and correlate with the lateral grinding movement required for an herbivorous diet. On the other hand, the grinding component was absent in dog and instead the chewing movements were primarily in a dorso-ventral direction, resembling the scissor like motion. The present hinge like movement of the TMJ in dog was due to 1) the relationship of the articular surface and the occlusal surface was relatively narrow, 2) the condylar process was found to be slightly above the occlusal plane, and 3) the joint is relatively distant from the last cheek tooth.
On the base of our observations in the present study, we conclude that the gross morphological differences and the arterial supply of the masticatory muscles in sheep and dogs and their spatial relationship to the temporo-mandibular joint influence greatly the movement of the joint and subsequently affect the feeding behavior in the two animal species.