HEAD AND NECK OF THE HORSE 49 



origin is by a strong and dense tendon from the whole of the facial crest 

 and from the zygomatic arch as far back as the mandibular joint. The 

 insertion of the muscle is to the lateral face of the mandible. Two 

 strata can be readily distinguished towards the origin of the muscle, but 

 these merge into one another at the mandibular attachment. Since 

 the superficial stratum arises from the malar bone and the maxilla only, 

 some of the fibres of the deep stratum are visible, without dissection, 

 near the mandibular joint ; and it will be noticed further that, in 

 direction, the fibres of the two strata cross each other. 



Dissection. — The masseter muscle must now be removed completely. 

 The transverse facial vessels should, in the tirst place, be traced through 

 the substance of the muscle. Then the reflex and buccinator veins must 

 be secured at their union with the facial vein. This permits the dis- 

 sector to obtain some idea of the thickness of the muscle, and should 

 enable him to perform its reflection without injury to underlying 

 structures. 



As the removal of the masseter proceeds, watch must be kept for its 

 nerve, which gains the muscle by traversing the notch between the 

 coronoid and condyloid processes of the mandible and lies close to the 

 bone in front of the mandibular joint. 



V. REFLEXA. — A portion only of the reflex vein is visible at this 

 stage of the dissection. Traced from its union with the facial, the vein 

 passes backwards underneath the masseter, along the upper border of 

 the buccinator muscle and the dorsal buccul glands. It disappears 

 from view between the mandible and the maxillary tuber, and will be 

 followed later when the mandible has been removed. 



There are two features of the reflex vein that should be noted at 

 this time. A vein of not more than moderate size at its anterior end, 

 it undergoes a fusiform dilatation just before it disappears from view. 

 The second characteristic upon which the dissector should satisfy 

 himself is the absence of valves. If the vessel be slit open for a short 

 distance and its interior be investigated with a probe, it will be found 

 that the passage of the instrument in either direction is unobstructed 

 by valves. 



V. BUCCINATORIA. — The buccinator vein, artery and nerve pursue 

 a curved course along the lower border of the buccinator muscle. Of 

 the three structures, the vein is most lateral, while the artery lies dorsal 

 to the nerve. 



Anteriorly the buccinator vein joins the facial opposite the termina- 

 tion of veins from the upper and lower lips. Posteriorly it approaches 

 the reflex vein, and, like this vessel, passes between the mandible and 

 the maxillary tuber. 



The buccinator resembles the reflex vein in being without valves and 

 4 



