76.Damage to which of the following structures might produce hair cell loss?

A.Basilar membrane B.Organ of Corti
C.Reissner's membraneD.Scala tympani
E.Scala vestibuli

ANS B.

Hearing is the detection of sound, which consists of pressure waves

conducted through air or water. The receptors of the cochlear duct provide us with a sense of hearing

that allows us to detect the quietest whisper and yet remain functional in a crowded, noisy

environment. The organ of Corti contains hair cells from the cochlear branch of the vestibulocochlear

nerve (CN VIII). These cells rest on the basilar membrane (choice A), which separates the scala

tympani (choice D) from the scala media. The hair cells are embedded in the tectorial membrane,

and movement of the basilar membrane below the cells causes the hairs to bend, which generates

action potentials. The tectorial membrane that lies on the hair cells does not form a boundary between

the different scala; the membrane separating the scala media from the scala vestibuli (choice E) is

Reissner's (vestibular) membrane (choice C).

1H

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75.Which cusp of the maxillary molar is NOT part of the trigon (primitive cusp triangle)?

A. Mesiolingual    B. Mesiobuccal

C. Distolingual     D. Distobuccal

ANS> C. In evolution, three cusps of the maxillary molar are considered to be the 

primary cusp triangle, and the original three cusps of the evolving molar. They form the trigon, and are 

the MB, ML, and DB cusps. The fourth cusp, the DL, is the smallest, is considered to be a minor cusp, 

and is known evolutionarily as the talon, or talon cusp. This cusp is less conspicuous as you go from 

first to second to third maxillary molar. 

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74. At which of the following ages does fetal movement first occur?

A. 1 month B. 2 months

C. 4 months D. 6 months E. 7 months


The correct answer is B. Neuromuscular development is sufficient to allow fetal movement in the

eighth week of life. Other features of Week 8 include the first appearance of a thin skin, a head as

large as the rest of the body, forward-looking eyes, appearance of digits on the hands and feet,

appearance of testes and ovaries (but not distinguishable external genitalia), and a crown-rump length

of approximately 30 mm. By the end of the eighth week, nearly all adult structures have at least begun to 

develop, and the fetus "looks like a baby." 

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