The arterial supply to head is primarily provided by branches from the common carotid artery. Ventral to the wing of the atlas the common carotid artery will branch into the internal and external carotid arteries with the occipital artery originating very near this termination.
The primary destination of the internal carotid artery is the cerebral arterial circle (formally the circle of Willis). Of interesting note, the internal carotid artery develops in the ruminant and cat but the extra-cranial portion of this artery will degenerate during the post-natal period. A remnant of the extra-cranial portion of this vessel can be seen as connective tissue strand.
The external carotid artery is the largest of the terminal branches of the common carotid artery. It can usually be observed coursing dorsally under the cover of the parotid salivary gland. Some species variation does exist when looking at the branches from the external carotid artery and only the primary ones are discussed in the following. In the ox and horse, the lingual and facial arteries branch from the external carotid artery as a combined trunk called the linguofacial artery and in the small ruminant only a lingual artery is present. Both the linguofacial and lingual arteries are rostro-ventrally directed branches. The following two braches are coursing dorsally; the caudal auricular and the superficial temporal. The superficial temporal artery marks the transition between the external carotid and the maxillary arteries. Both the transverse facial (all species) and the cornual arteries of horned animals are braches from the superficial temporal.
The facial artery, when present, breaks from the linguofacial trunk medial to the mandible and will course to the lateral surface of the mandible rostral to the masseter muscle, in the vascular notch. In the horse, this is an area where the pulse can be taken. Once the facial artery is lateral to the mandible the following braches can be found: inferior and superior labial arteries, lateral nasal, and the dorsal nasal (only in the horse). The continuation of the facial artery after either the lateral or dorsal nasal arteries branch is the angularis oculi. Some of these braches may be difficult to identify in dissection since our specimens are not injected.
The maxillary artery supplies many of the deep structures of the head and much of the tissue in the retrobulbar area. One branch that is found in all species is the mandibular artery, which can generally be observed very near the origination of the maxillary. The maxillary artery transitions to the infraorbital at the maxillary foramen.
For the most part our naming of the venous returns mirrors the arterial names with a few variations. One important point in the small ruminant is the presence of a facial vein. The branches of this vessel will be similar to that of the ox. Another unique aspect of venous return from the head is the presence of deep venous plexuses and sinuses. The venous plexuses are best developed in the ox and the venous sinuses in the horse. Both structures are found deep to the masseter muscle but lateral to the maxilla. It is thought that contractions of the masseter helps to ?pump? blood back centrally when the animal has its head down in a grazing position (ie the contracting muscle is increasing the venous pressure since it is squeezing the plexus or sinus against the maxilla).