SUPERFICIAL VESSELS
The facial artery and vein enter the face in company with the parotid duct (Figure 18-7/8). The artery is easily found where it is in contact with the bone and is convenient for taking the pulse (see Figure 18-40/7).
The artery can best be palpated just before it crosses the lower border of the mandible (on the medial side of the mandible). The artery then ascends along the rostral margin of the masseter before terminating in divergent branches; although the pattern of collateral and terminal branches varies, it is usually possible to identify inferior and superior labial, lateral and dorsal nasal, and angularis oculi arteries.The arrangement of the veins is similar, and their pattern may be visible in life in thin-skinned horses. Certain of the tributaries turn caudally, deep to the masseter, to anastomose with other veins of the head. The most dorsal connection, the transverse facial vein (see Figure 18-9/4), joins the superficial temporal vein. The rostral part lies deep to the masseter, but it then penetrates the muscle; in the caudal part of its course it lies superficially and follows the ventral edge of the zygomatic arch. This caudal stretch is accompanied by an artery (an alternative site for examination of the pulse) and a nerve. Another site for pulse taking is the subcutaneous segment of the masseteric artery (see Figure 18-7/12).
The second connection, the deep facial vein (see Figure 18-9/5), burrows below the masseter and perforates the periorbita before passing through the orbital fissure to join the cavernous venous sinus within the cranial cavity. Two features of this vein are believed to possess functional significance. The discharge into the cavernous sinus contains relatively cool blood drained from the hard palate and nasal cavity; because the sinus envelops the internal carotid artery, this cools the arterial blood passing to the brain, where the temperature is monitored as part of the heat control mechanism. Secondly, an expansion of the vein deep to the masseter may form the basis of a pumping mechanism. It is liable to compression by the masseter, and it is asserted that this helps prevent stagnation of the venous return from the lowered head of the grazing animal.
There is a similar expansion on the third connection, the buccal vein (see Figure 18-9/6), which also runs deep to the masseter to join the superficial temporal tributary of the maxillary vein.
There are two superficial groups of lymph nodes. The parotid group under cover of the rostral part of the parotid gland is not usually palpable unless enlarged. The second group comprises numerous mandibular nodes arranged in a spindle within the intermandibular space. Together with their contralateral fellows, these nodes form a forward-pointing V that is always very distinctly palpable (see Figure 18-39/2). The course of the lymph flow is dealt with later (p. 531).