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I now have all the information needed for a thorough, easy-to-understand essay on the vocal cords. Here is the complete answer:

The Vocal Cords - A Complete Easy Guide

Anatomy of the larynx showing vocal cord positions during phonation, abduction, and whisper

Figure: (A) Larynx anatomy showing key cartilages and muscles. (B) Vocal cord positions during different voice activities - from full open breathing to phonation (speech).

1. What Are the Vocal Cords?

The vocal cords (also called vocal folds) are two bands of specialized tissue stretching across the inside of the larynx (your "voice box"), which sits at the top of your windpipe (trachea). They are not cords at all in the rope sense - they look more like lips or folds of tissue.

There are actually two sets of folds:

True vocal folds (lower) - these vibrate to make sound
False vocal folds / vestibular folds (upper) - mainly protective, do not usually vibrate for normal speech

2. Where Are They Located? (The Larynx)

The larynx contains three unpaired cartilages (epiglottis, thyroid cartilage, cricoid cartilage) and three paired cartilages (arytenoid, corniculate, cuneiform). The vocal folds attach at the front to the thyroid cartilage and at the back to the arytenoid cartilages - and it is the movement of the arytenoids that opens and closes the vocal folds.

The space between the two vocal folds is called the glottis. The larynx has three regions:

Supraglottis - above the folds (includes epiglottis)
Glottis - the folds themselves and the 1 cm below
Subglottis - below, down to the cricoid cartilage

(Sabiston Textbook of Surgery, p. 1361)

3. Layers of the Vocal Fold (Microarchitecture)

This is the key to understanding how the vocal cords work. From outside to inside, each fold has:

Layer	Contents
Epithelium	Stratified squamous cells, ~50 microns thick
Superficial lamina propria (Reinke's space)	Loosely arranged collagen and elastin - acts as a shock absorber, allows the mucosal "wave"
Intermediate lamina propria	Predominantly elastic fibers
Deep lamina propria	Dense collagen fibers
Vocalis muscle (thyroarytenoid)	The muscular "body" of the fold

The intermediate + deep layers together = the vocal ligament.

Hirano (a pioneer researcher) described this beautifully as a "body-cover" model: the vocalis muscle is the relatively stiff "body," while the epithelium + Reinke's space form the soft "cover" that ripples freely over it. This free rippling is what gives the voice its richness and range. If the Reinke's space is scarred (e.g., after surgery or injury), voice quality drops dramatically. (Cummings Otolaryngology, p. 1107-1108)

4. How Do the Vocal Cords Make Sound? (Phonation)

Sound production involves three steps: breath power, vibration, and resonance.

Step 1 - Air Power (Subglottic Pressure)

You breathe in, then begin to breathe out. The air below the vocal folds builds up pressure (subglottic pressure).

Step 2 - The Vibration Cycle (Myoelastic-Aerodynamic Theory)

This is the widely accepted explanation for how sound is made (van den Berg, 1950s):

The vocal folds are held near the midline (adducted/closed)
Air pressure from below pushes the folds apart
As air rushes through the narrow gap, the Bernoulli effect (air moving fast = lower pressure) and the elastic recoil of the folds pull them back together
The folds snap shut, pressure builds again - and the cycle repeats hundreds of times per second

The mucosal wave starts at the lower edge of the fold and travels upward - as the bottom is closing, the top is still opening. This is the "two-mass model" of vibration. (Cummings Otolaryngology, p. 1023-1024)

"Vocal fold structure determines whether the resulting vibration is periodic or chaotic." - Cummings Otolaryngology

Step 3 - Resonance

The raw buzzing sound is shaped into speech by the mouth, tongue, lips, soft palate, nose, and throat cavities. (Guyton & Hall, p. 502)

5. Controlling Pitch and Volume

What changes	Effect
Increase tension in the fold (cricothyroid muscle stretches the fold)	Higher pitch
Shorten the fold	Higher pitch
Reduce mass of the fold	Higher pitch
Relax the vocalis muscle	Lower, richer "chest voice"
More air pressure (deeper breath, more effort)	Louder voice
Falsetto (very tight, thin folds, only top edges touch)	Very high pitch; mucosal wave disappears

The cricothyroid muscle (the main pitch-control muscle) is the only intrinsic laryngeal muscle controlled by the external (superior) laryngeal nerve. All other intrinsic muscles are controlled by the recurrent laryngeal nerve (RLN). (Guyton & Hall, p. 501-502)

6. Positions of the Vocal Folds

As seen in the diagram above, the folds take different positions:

Full abduction (wide open) - quiet breathing
Gentle abduction - deep breathing
Adduction/phonation (nearly closed) - speech and singing
Stage whisper - folds apart but tightly tense, turbulent airflow
Loud whisper - intermediate position

7. What Can Go Wrong? (Common Disorders)

Disorder	What happens
Vocal nodules	Callus-like thickenings at the midpoint, from vocal overuse ("vocal overdoers") - e.g., teachers, singers
Vocal polyps	Soft, fluid-filled swellings, usually one-sided; from acute vocal injury
Vocal cord paralysis	One or both folds cannot move, usually from recurrent laryngeal nerve damage (e.g., thyroid surgery, lung cancer); causes breathy, weak voice
Reinke's edema	Fluid accumulation in Reinke's space; common in smokers; causes deep, rough voice
Laryngitis	Inflammation from infection or reflux; swelling prevents normal vibration
Vocal cord scar	Loss of the pliable Reinke's layer; permanent voice change
Papillomatosis	Wart-like growths from HPV infection

(Scott-Brown's Otorhinolaryngology; Cummings Otolaryngology)

8. How Are Vocal Cord Problems Diagnosed?

Laryngoscopy (flexible or rigid camera): the standard way to directly view the folds
Videostroboscopy: uses a strobe light to slow down the apparent vibration and assess the mucosal wave - the most useful tool for diagnosing benign fold disorders
High-speed video: true slow-motion capture of vibration (expensive, research use)
Acoustic analysis: computerized measurement of voice quality

Quick Summary

VOCAL CORDS
    │
    ├── Location: Larynx (between thyroid & arytenoid cartilages)
    ├── Layers: Epithelium → Reinke's space → Vocal ligament → Vocalis muscle
    ├── How sound is made: Myoelastic-aerodynamic theory
    │       Air pressure pushes folds apart → Bernoulli + elastic recoil closes them
    │       → Vibration → Resonance in vocal tract → Speech
    ├── Pitch control: Tension (high pitch), relaxation (low pitch)
    └── Common problems: Nodules, polyps, paralysis, polyps, scar

The vocal folds are a marvel of biological engineering - two tiny tissue bands, each only about 15-20 mm long in an adult, yet capable of producing the full range of human speech, singing, and expression by vibrating up to 1000 times per second.

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