Anatomy, Morphology and Physiology practice test 2, every answer explained

Answer: B. The portion on show in the mouth, which rarely equals the enamelled part

Clinical crown measures what is exposed: whatever stands visible in the mouth on the day of the examination, and it seldom lines up with the area enamel covers. That enamelled area is the anatomic crown, a fixed quantity that does not change as gum tissue rides up or down the tooth.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “3-5. PARTS OF TEETH”. U.S. government work, public domain.

Answer: D. The motor root drives the muscles of mastication; the sensory root brings feeling in from the head and the face

The fifth nerve is mixed, and the manual splits the work cleanly: its motor root sends impulses to the chewing muscles while its sensory root brings sensation back from the structures of the head and face. Nothing there gives the nerve the muscles of the tongue or of the lips, and swapping the two roles around — sensation to the chewing muscles, movement from the face — is the commonest way this one is misremembered.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “2-10. NERVE SUPPLY OF THE JAWS AND TEETH”. U.S. government work, public domain.

Answer: A. The capillaries, the billions of tiny vessels that the arteries branch into

The exchange happens in the capillaries, the billions of tiny vessels the arteries branch into, because their walls are thin enough for food and oxygen to cross out of the blood and for carbon dioxide and waste to cross back in. Arteries and veins are transport vessels rather than exchange vessels: arterial walls are elastic and muscular, and venous walls, though thin, are collapsible and carry blood back to the heart behind paired valves. Lymphatic vessels drain lymph, not oxygenated blood.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “1-9. CIRCULATORY SYSTEM”. U.S. government work, public domain.

Answer: D. The back third rests on no bone at all and is built mainly of muscle under its lining

What makes the front of the palate immovable is the bony plate beneath its covering; the back of the palate has nothing of the kind. It is a curtain of muscle clothed in mucous membrane, which is exactly why it can be lifted, and why its rear edge hangs free instead of ending against bone. Neither thinned bone nor cartilage is present there.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “3-4. THE ORAL CAVITY”. U.S. government work, public domain.

Answer: A. The mastoid process, a projection of the temporal bone

The mastoid process is the part of the temporal bone felt as a knob behind the ear. It contains air cells that open into the middle ear and gives attachment to several muscles that move the head. The zygomatic process of the same bone runs forward from above the ear canal to meet the cheekbone, so it is felt in front of the ear instead. The coronoid and condyloid processes are the two projections on the upper edge of the ramus of the lower jaw, and neither is part of the cranium.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “2-3. CRANIUM”. U.S. government work, public domain.

Answer: B. In the maxilla, the commonest being a mesiodens between the central incisors

Both halves of the answer come from one sentence: the upper jaw holds most of them, and the commonest sits in the midline between the upper central incisors. Moving it to the lower jaw, or naming a different extra tooth, breaks one half or the other.

Source: Oral and Maxillofacial Pathology (subcourse MD0511), U.S. Army Medical Department Center and School; section “2-18. SUPERNUMERARY TEETH”. U.S. government work, public domain.

Answer: C. Closer to the crown than on any other tooth in either arch

This tooth's roots part company nearer the crown than the roots of any other tooth do, so the split is visible high up. Splitting near the tips, or at the midpoint like other molars, would put the division much lower, and the roots certainly do divide.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “4-23. MANDIBULAR FIRST MOLAR”. U.S. government work, public domain.

Answer: C. Their leading, or incisal, edges, one set against the other

The comparison is made at the cutting edges of the two sets of front teeth. A gum margin, a point of contact with a neighbour and a root tip are all real landmarks, but none of them is what this measurement is read from.

Source: An Assessment of the Occlusion of the Teeth of Youths 12–17 Years, United States (Vital and Health Statistics, Series 11, No. 162), National Center for Health Statistics, 1977; section “Vertical Overbite or Openbite”. U.S. government work, public domain.

Answer: A. A groove running off the chewing surface and down the tongue side

A groove divides the pair and carries over the edge of the chewing surface onto the tongue side of the crown. A ridge joining the tips would fuse them rather than separate them, a central pit is a different feature, and the subcourse does describe a divider, so treating the cusps as meeting directly is wrong.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “4-22. MANDIBULAR SECOND BICUSPID”. U.S. government work, public domain.

Answer: B. About four fifths of it, leaving little for the tongue-side cusp

The cusp on the cheek side dominates the surface, taking roughly four fifths and leaving the tongue-side cusp very small, which is what gives this tooth its lopsided look. An even split, a reversed split, or a single cusp would all describe a different tooth.

Source: Dental Anatomy and Physiology (subcourse MD0501), U.S. Army Medical Department Center and School; section “4-21. MANDIBULAR FIRST BICUSPID”. U.S. government work, public domain.