The most prominent feature distinguishing the porcine glottis from that of other mammalian species, including humans, is the fact that its vocal ligament is split longitudinally into two parts, i.e. a cranial and a caudal part.
Both parts of the split vocal ligament originate from the cricothyroid ligament at the ventral part of the thyroid cartilage and insert on the vocal process of the arytenoid cartilage (WAIBL 2004).
With respect to this bi-parted organisation of the porcine vocal ligament, special attention is to be paid to the laryngeal mucosa: The mucosa forms a lateral recess, i.e. ventriculus laryngis (INTERNATIONAL COMMITTEE ON VETERINARY GROSS ANATOMICAL NOMENCLATURE 2005), also called ventriculus laryngis lateralis (KÖNIG and LIEBICH 2009). However, in pigs, the opening of this pouch lies between the cranial and the caudal part of the vocal ligament (WAIBL 2004), and not between the vocal ligament and the vestibular ligament (as it does, e.g., in dogs, horses, or humans; see WAIBL [2004], STANDRING [2005]).
These unique features – bi-partition of the vocal ligament with a lateral outgrowth of laryngeal mucosa between them – have contributed to some inconsistent definitions of what the so-called vocal fold really is in pigs.
In the past, BUROW (1902) and PRODINGER (1940) suggested that there was only one single vocal fold (one on each side – right and left – of the larynx), which contained the two parts of the split vocal ligament. In contrast, the vocal fold described by WAIBL (2004) contains only the caudal part of the split vocal ligament.
In recent years, authors have proposed to distinguish two vocal folds (i.e. two on each side of the larynx): ALIPOUR and JAISWAL (2008, 2009) referred to them as the superior and inferior vocal fold, while – respecting the quadruped organisation of the porcine body – KOCH et al. (2010) spoke of the cranial and caudal fold of the
porcine glottis. Both of these folds are part of the glottis, which is the vocal apparatus of the larynx (SCHALLER 2007), and therefore should possess phonatory properties;
this has been demonstrated by ALIPOUR and JAISWAL (2008, 2009). Consequently, the folds’ assignment as a cranial vocal fold and a caudal vocal fold – as in a previous study (LANG et al. 2013 b) – appears equally appropriate.
In all parts of the following text, the simplified terms ‘cranial fold’ (CraF) and ‘caudal fold’ (CauF) are applied.
The porcine vestibular ligament, also called ligamentum vestibulare (INTER-NATIONAL COMMITTEE ON VETERINARY GROSS ANATOMICAL NOMEN-CLATURE 2005) or ligamentum ventriculare (BUROW 1902; PRODINGER 1940) lies far cranial of the vocal ligament. It extends from the base of the epiglottis to the lateral surface of the arytenoid cartilage up to its corniculate process (WAIBL 2004).
Its structure is rather membrane-like than string-like, and, as such, differs significantly from its human equivalent. The vestibular ligament is not involved in the formation of a vestibular fold in pigs (WAIBL 2004); this means that there is no vestibular fold in the porcine larynx (BUROW 1902; PRODINGER 1940; WAIBL 2004).
According to WAIBL (2004), the porcine larynx contains a fibroelastic membrane: It originates at the lateral parts of the cricothyroid ligament and ends on the caudal part of the vocal ligament. WAIBL (2004) equates it with the human conus elasticus.
However, this assumption does not appear appropriate, as the fibroelastic membrane in humans has two components (STANDRING 2005): Firstly, the conus elasticus (in inferior position); and secondly, the membrana quadrangularis (in superior position).
WAIBL (2004) does not make reference to such a membrana quadrangularis as part of the fibroelastic membrane in the porcine larynx. Nevertheless, he does describe a fibrous sheet located between the vestibular ligament and the cranial part of the split vocal ligament (i.e. supraglottically), which extends from the thyroid lamina to the ventral border of the arytenoid cartilage. In principle, this fibrous sheet appears to resemble the human membrana quadrangularis, except that it is smaller in dimension (it does not line the entire supraglottic wall of the larynx).
Lateral to this sheet/membrane lies the lumen of the laryngeal ventricle. Next to its opening, it is shaped like an oval duct, but then continues into a pouch-like recess reaching up to the level of the vestibular ligament (PRODINGER 1940).
The thyroarytenoid muscle is the main muscle in the porcine glottis (WAIBL 2004). Its broad and flat body extends from the ventro-caudal part of the thyroid cartilage to the arytenoid cartilage (WAIBL 2004). The muscle is located inside the cartilaginous skeleton of the larynx, but lies lateral to the split vocal ligament, and also lateral to the laryngeal ventricle (PRODINGER 1940). As the lumen of the laryngeal ventricle extends cranially (past the cranial part of the split vocal ligament), it ‘pushes in between’ the cranial part of the split vocal ligament and the thyroarytenoid muscle (PRODINGER 1940).
Due to this anatomical situation, there is no muscular component in the cranial fold (CraF) of the porcine glottis. The thyroarytenoid muscle lies directly lateral to the caudal part of the split vocal ligament and thus constitutes a large part of the caudal fold (CauF).
Unlike in other species (e.g. in horses or dogs), the thyroarytenoid muscle of pigs shows no sign of a subdivision into a vocal muscle and a ventricular muscle (PRODINGER 1940; WAIBL 2004).
In summary: What seems to resemble the ventricular fold in humans is in fact the cranial fold of the porcine glottis. This fold is supposed to be the main oscillator in the pig’s glottis (ALIPOUR and JAISWAL 2008, 2009). Understanding this striking anatomical and functional difference is essential for avoiding misinterpretations and confusion when comparing the results of histological and phoniatrical studies on pigs and other mammalian species, including humans.
Table 1 gives a survey on synonyms and definitions of terms related to the human and porcine glottis. They were applied in the literature of which reference is made in the text of this thesis.
A further, more detailed description of the human glottis is to be found in Annex 9.1:
Anatomy and terminology of relevant features of the human glottis with regard to species differences.
Tab. 1. Synonyms and definitions of terms related to the human and porcine glottis;
data in brackets [ ] are explained at the end of the table.
Official term
Synonyms Description
Vocal fold [TA]
Human Plica vocalis [TA], vocal cord [1], true vocal cord [1], Stimmfalte* [2],
Stimmlippe* [3]
Vocal ligament and vocal muscle covered by mucosa
Porcine Cranial and caudal (vocal) fold [4], CraF and CauF [4], superior and inferior vocal fold [5]
Cranial and caudal part of the vocal ligament and
cartilage; upper free edge of the conus elasticus
Porcine Ligamentum vocale [NAV], Stimmband* [6]
Ligament between cricothyroid ligament and arytenoid
cartilage; split into a cranial and a caudal part
Vestibular fold [TA]
Human Plica vestibularis [TA], false vocal fold [1], ventricular fold, Taschenfalte* [2], Vorhoffalte* [7]
Vestibular ligament covered by mucosa
Porcine Term does not apply (see explanation on the right)
Absent
* Term commonly used in German language
Official
A slit between the vestibular fold and the vocal fold opens into a fusiform recess; it extends
extends cranially to the level of the vestibular ligament
Main muscle lateral to the vocal fold, the cricovocal membrane, and the laryngeal ventricle Porcine Musculus thyroarytenoideus
[NAV], musculus
thyreoarytaenoideus [NAV]
Main muscle lateral to the vocal folds and the laryngeal ventricle
Vocal muscle [10]
Human Vocalis [TA], musculus vocalis [TA]
Medial portion of the thyro-arytenoid muscle; lateral to the vocal ligament
Porcine Term does not apply (see explanation on the right)
The thyroarytenoid muscle is not subdivided, therefore there is no separate vocal muscle
* Term commonly used in German language
[TA] Terms listed in Terminologia Anatomica (FEDERATIVE COMMITTEE ON ANATOMICAL TERMINOLOGY 2011), i.e. anatomy of humans
[NAV] Terms listed in Nomina Anatomica Veterinaria (INTERNATIONAL COMMITTEE ON VETERINARY GROSS ANATOMICAL NOMEN-
CLATURE 2005)
[1] STANDRING (2005)
[2] SOBOTTA and WELSCH (2009) [3] DRUNCKER and KUMMER (2008) [4] LANG et al. (2012 a)
[5] ALIPOUR and JAISWAL (2009) [6] WAIBL (2004)
[7] LIPPERT (1990)
[8] BUROW (1902)
[9] PRODINGER (1940) [10] REUTER and REUTER (1996) [11] KÖNIG and LIEBICH (2009) [12] CLEMENTE (1997)