Tree shrew major vessels
Photo and text: Terence Mitchell
The following vessels are difficult to dissect because they are very small (< 1 mm), and are also imbedded in various layers of soft tissue, muscle, fascia, and even glandular tissue. Start from obvious origin or terminal branches, and separate the vessels from the surrounding tissue with the sharp probe. Locate the course of as many of the vessels as possible before removing any large pieces of tissue. Use the small forceps to remove any tissue.
Common carotid - From the aorta the common carotid runs cranially in the neck before giving off an occipital branch. In Figure 1 the stump of this branch is visible on the common carotid. Superiorly, the internal carotid branches off and enters the skull through the carotid foramen. The external carotid then continues on and gives off two more major branches: a common branch for several arteries, and the lingual artery. The former splits into the facial artery and another common trunk for the posterior auricular, superficial temporal, and maxillary arteries. The facial artery runs medial to the masseter and then sends off several vessels to the superficial muscles of the face. The lingual artery dives deep to the digastric muscle to supply the muscles of the tongue. The hypoglossal nerve is associated anatomically with several branches of the internal carotid. The vagus nerve runs with the carotid artery in the carotid sheath.
Jugular vein - Before it splits into the internal and external jugular veins, the common jugular surrounds the clavicle in a venous ring and at this point accepts several branches, which drain most of the external tissues of the head. This ring makes it difficult to distinguish the convergence of the subclavian vein and the jugular. However, in Figure 2 [Editor's note: Figure 2 is not included on this page] just superior to this ring the internal jugular joins the external jugular after descending from the skull. Next, the larger external jugular accepts a robust postglenoid vein, which drains most of the blood from the brain and then exits the brain case through the postglenoid foramen, anterior to the auditory bulla. Another smaller branch of the external jugular accepts the lingual vein and facial vein. The lingual vein originates deep to the digastric muscle and drains the muscles of the tongue. The facial vein drains the superficial muscles surrounding the cheek, eye, and part of the forehead. Although they are not shown in Figure 2, I also located the superior and inferior labial veins, which converge on the facial vein and drain the blood from the skin surrounding the lips.
The branches of the common carotid are frequently used as characters in phylogenetic analyses of many mammalian taxa [1]. Characters related to the common carotid and many other cranial characteristics have been used to associate tree shrews and primates. However, many of these characters are irrelevant, primitive, or don't take into account a suitable spectrum of taxonomic and individual diversity [2]. There are several primitive traits of the carotid, such as retention of a temporal ramus and ramus inferior of the stapedial artery. Another primitive trait, retention of the opthalmic artery, links primates with tree shrews. However, these symplesiomorphies cannot be used to determine phylogenetic relationships. Three additional traits, encapsulation of the promontory artery in a bony tube, encapsulation of the stapedial artery in a bony tube, and loss of the medial internal carotid artery are possible synapomorphies between primates and tree shrews. If the first two traits are assumed to be valid synapomorphies, then primates must be more closely related to tree shrews than they are to plesiadapoids. This suggests that these characters are convergences. Therefore, we are left with loss of the medial internal carotid as the only possible synapomorphy linking tree shrews and primates. In conclusion, there is little persuasive evidence for tupaiid-primate affinities in the common carotid artery.
1 Asher, R.J. 2001. Cranial Anatomy in Tenrecid Insectivorans: Character Evolution Across Competing Phylogenies. Am. Mus Novitates 3352: 54 pp.
2 Cartmill, M. and R.D.E. MacPhee. 1980. Tupaiid Affinities: The Evidence of the Carotid Arteries and Cranial Skeleton. In: Luckett, W.P., ed. Comparative biology and evolutionary relationships of tree shrews: 95-132. New York: Plenum Press.
Madeira, M.C. and Watanabe. 1973. Superficial Venous Arrangement of the Face
and Neck of the Tufted Capuchin, Cebus apells. J. Morphol. 140:397-404.
Dom, R., B.L. Fisher, and G.F. Martin. 1970. The Venous System of the Head
and Neck of the Opossum (Didelphis virginiana).
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