This SI unit is named after Heinrich Hertz. As with every International System of Units SI unit named for a person, the first letter of its symbol is upper case Hz. However, when an SI unit is spelled out in English, it is treated as a common noun and should always begin with a lower case letter hertz —except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.
Brief Overview of Ear Structures and Function Anatomy In reptiles with external ear structures, the tympanic membrane is visible, either nearly contiguous to the surface of the skin as with iguanids such as Hertz case green iguanaor recessed deeper into the head as with some scincids, such as the blue-tongue skink, and agamids, such as the bearded dragon.
The tympanic membrane covers the middle ear cavity. In fact, it is the outer boundary of this cavity which is linked, on its other side to the pharynx and eustachian tube. In general, the inner boundary of the middle ear cavity has two openings.
There is a round one, covered by a thin membrane, and, farther back towards the neck, an oval opening which is uncovered. The stapes crosses the middle ear cavity, from the inside of the tympanic membrane, its inner end fitted inside the oval opening.
The outer end of the stapes has a cartilage cap which comes into contact with the tympanic membrane. In Hertz case reptiles, this cartilage, called the extrastapes, is attached to the quadrate, the primary support of the lower jaw.
Beyond the round and oval openings of the middle ear cavity is the inner ear cavity. Here are located the organs related to balance the semicircular canals, utricle, and saccule and hearing cochlear duct. The cochlear duct and the saccule are both suspended in perilymphatic fluid; the cochlear is also filled with this fluid.
The inside of the duct has two specialized regions, the papilla basilaris and the smaller macula lagenae. Both of these areas are actually clusters of sensory cells. These areas also have cilia which are embedded in a membrane within the cochlear duct.
These sensory cells give rise to the auditory nerve the VIIIth cranial nerve. Function Airborne vibrations are picked up by the tympanic membrane. Substrate ground or other conducting surface on which the reptile is in close contact vibrations are detected by the quadrate.
The tympanic membrane or quadrate vibrates, in turn vibrating the extrastapes and thus the stapes. This causes the vibrations to be conducted through the middle ear cavity, through the windows to the fluid-filled inner ear cavity housing the sensitive cochlear duct, whose sensory clusters then transmit the information along the auditory nerve.
This means, of course, that even without a surface or subsurface-mounted tympanic membrane, many "earless" reptiles can indeed "hear", though to varying degrees.
The tympanic membrane is absent in many fossorial burrowing and semi-fossorial lizards, such as the legless Anniella, as well as in other reptiles, such as the tuatara, amphisbaenians, and, of course, snakes.
There is a great variation in the tympanic membrane and sensitivity of the inner ear amongst those lizards and chelonians with tympanic membranes "eared". Morphological variations include the depth of the structures from the surface of the head, the sizes of the structures, thickness of the various membranes, etc.
Some eared lizards, as mentioned above, have surface-mounted tympanic membranes.
Others have a recessed membrane, rather like the human tympanic membrane is recessed inside the head. Whereas our ears are marked by a rather visible flap of cartilaginous skin which helps conduct vibrations into our ear, other eared reptiles don't have the significant structure as do ours, though some species have angled recesses, or scales that grow farther out from the head just in front cranially of the recess, which may serve to channel vibrations or, more likely, protect the recessed membrane further from getting poked by sharp objects such as twigs and claws.
Crocodilians and geckos have a small muscle that is next to or upon the stapes, the stapedius, which may function in the way the mammalian stapedius muscle does: However, given the number of humans whose hearing has been permanently impaired by listening to loud music, or loud engine or other machinery noise, one should not assume that the stapedius provides full protection against such damage in humans, nor in those reptiles who have this muscle.
In the tuatara, the stapes is longer, coming into contact with the quadrate as well as the hyoid and squamosal. Their middle ear cavity is filled with loose tissue, mostly adipose. Crocodilians, on the other hand, have a complex of bony air-filled passages and a branching eustachian tube.
Amphisbaenians show at least two variations of extrastapes-stapes morphology, both connecting more closely with the lower jaw.
In those reptiles lacking the tympanic membrane, what would be the middle-ear cavity is divided, by a bony partition, into two chambers. The extrastapes passes through the outer chamber, into which opens the eustachian tube.
The inner chamber is called by different names, depending upon whose skull it is in: In many reptiles, including turtles, snakes, and amphisbaenians, the round window leading to the inner ear, is missing. Instead, other ways have evolved to dissipate the vibrations in the perilymphatic fluid.
In crocodilians, the cochlear duct is elongated and differs in other ways amongst this group. The cochlear duct in turtles differs from other reptiles in that that the two sensory areas are not as far apart from one another.
In studies of the cochlear duct's papilla basilaris macula lagenae, as well as their cilia and nerve fibers, the patterns found are often so significant that they can help trace taxonomic and phylogenetic relationships.
Some of the differences point to other functions, such as the enlarged papilla basilaris in those geckos that vocalize, an area that are larger than the same area in their more fossorial cousins.Visit a Hertz Car Sales dealership or Rent2Buy location near you to buy a used car.
We offer financing, trade-ins, vehicle protection plans, three day test drives and much more. The hertz (symbol: Hz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second. It is named for Heinrich Rudolf Hertz, the first person to provide conclusive proof of the existence of electromagnetic waves.
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