The morphology and innervation of the maxilla, mandible, and teeth are known to vary with the change of feeding habit in mammals. However, it is not known if the masticatory muscles show parallel changers. The purpose of this study was to provide anatomical information about the course and distribution of the mandibular nerve and its branches in the masticatory muscles. The heads of five sheep were fixed by perfusion with 10% formalin. The course and branches of the mandibular neurovascular bundle were examined bilaterally by dissection under a microscope. 1. The masseter muscle of the sheep is the largest of the masticatory muscles, and it consists of four layers. These are the superficial, middle, deep and deeper layers. They are supplied by the masseteric nerve branching from the mandibular nerve. 2. The temporal muscle in the sheep is much smaller than in dog and human. It is divided into two layers by the coronoid process : the superficial layer attaches to the lateral surface of the coronoid process ; the deep layer attaches to the medial surface. The insertion of the deep layer extends to the anterior edge of the mandibular foramen. The nerve that innervates the deep layer is supplied by the posterior deep temporal nerve branching from the mandibular nerve ; the nerves that innervate the superficial layer originate from the anterior and posterior superficial temporal nerve. The anterior superficial temporal nerve branches from the masseteric nerve, and the posterior one from the posterior deep temporal nerve. 3. The lateral pterygoid muscle is the smallest masticatory muscle. It attaches to the antero-medial surface of the condyle and the medial surface of the articular capsule. The muscle is supplied by the lateral pterygoid nerve branching from the buccal nerve. The nerve undergoes some anastomoses with its own branches to form an intramuscular network. 4. The medial pterygoid muscle of the sheep is much bigger than in dogs and human beings. It is supplied by the medial pterygoid nerve originating from the mandibular nerve. The medial pterygoid muscle is divided into two layers based on its innervation. Considering its innervation, the lateral movement of the mandible depends on cooperation between the lateral pterygoid muscle, the posterior part of the third layer of the masseter muscle, and the anterior part of the superficial temporal muscle.
