The Larynx

Hello :) In this post, we'll discuss the anatomy of the larynx including the airway itself, its cartilages, ligaments and muscles.

Airway

The laryngeal cavity is known as the aditus larynges. The inlet to this cavity leads into a wide chamber known as the vestibule of the larynx. The middle portion of the larynx is called the glottis and is surrounded by the arytenoid cartilages dorsally and the vocal folds ventrally. This narrows to form the glottic cleft which is a narrow passageway that runs towards the trachea. In the dog and horse, as the glottic cleft moves caudally, it opens laterally to form two lateral laryngeal ventricles. 

Cartilages

The larynx contains four types of cartilages: the epiglottis, the thyroid, the cricoid and arytenoids. All are unpaired except for the arytenoids.

The epiglottis folds back to cover the glottis (the opening to the larynx) during swallowing. The cricoid is ring-shaped and is the only cartilage to surround the larynx completely. The thyroid forms the “Adam’s apple” in people. Rostrally, the thyroid cartilage articulates with the thyroid bone of the hyoid apparatus, and caudally it articulates with the arch of the cricoid. The arytenoids control the vocal folds by rotating.

Ligaments

The cricothyroid ligament connects the ventrocaudal part of the thyroid cartilage to the ventral arch of the cricoid cartilage.

The cricotracheal ligament connects the larynx with the first tracheal cartilage.

The vocal ligament extends from the vocal processes of the arytenoids to either side of the body of the thyroid. It forms the basis of the vocal fold.

In animals that have a vestibular fold, a vestibular ligament exists rostral to the vocal ligament.

Muscles

Intrinsic Musculature

The intrinsic muscles contract to narrow or widen the glottic cleft which tenses or relaxes the vocal folds. All are innervated by the recurrent laryngeal nerve except for the cricothyroid muscle which is innervated by the cranial laryngeal nerve.

The cricothyroid muscle extends between the lateral surfaces of the cricoid and thyroid cartilages. It is innervated by the cranial laryngeal nerve. It tenses the vocal folds when it contracts.

The dorsal cricoarytenoid is the main abductor of the vocal folds and widens the glottic cleft. It extends from the dorsal surface of the cricoid to the muscular process of the arytenoid. A lateral cricoarytenoid also exists and this extends from the cricoid arch and the muscular process of the arytenoid cartilages. When this contracts, it narrows the glottic cleft.  

The transverse arytenoid connects the muscular processes of each arytenoid to each other. It adducts the two arytenoid cartilages and narrows the glottic cleft.  

The thyroarytenoid extends from the base of the glottis and thyroid cartilage to the muscular and vocal processes of the arytenoid cartilage. They increase the tension of the vocal folds and narrow the glottic cleft. 

And that's it! See you next time :)

The Palate, Nasal Conchae and Meatuses, the Vomeronasal Organ and Guttural Pouch

Hi, in this post we’ll take a look at the anatomy and function of the hard and soft palate, nasal conchae, nasal meatuses, the vomeronasal organ, the incisive duct, and the guttural pouch.

The Palate

The palate forms the roof of the oral cavity and the floor of the nasal cavity. It is divided into two sections: the hard palate and the soft palate.

The hard palate is formed from the palantine, incisive and maxillary bones of the skull and is covered by a highly vascularised layer of mucosa. It is surrounded by the upper teeth on its rostral and lateral sides and contains a series of transverse ridges known as rugae which help direct food towards the molars during mastication (chewing).The hard palate contains the vomeronasal organ.

 The caudal border of the palate is composed of a muscular tissue called the soft palate which separates the oropharynx and nasopharynx and opens to the laryngopharynx caudally. It acts as a valve during swallowing that ensures that water/ingesta flow into the oesophagus and not the trachea. In the horse, it completely separates the nasal cavity from the oral cavity.

Nasal Conchae

The median nasal septum divides the nasal cavity into two compartments. The septum has bony and cartilaginous compartments which form the nasal cartilages. The nasal conchae are scroll-like structures located in the lateral regions of the compartments of the nasal cavity and increase the surface area of the nasal mucosa for olfaction.

The most dorsal and also the longest concha is the dorsal nasal concha which extends the furthest into the nasal cavity from the cribriform plate. A ventral nasal concha also exists and this is a separate bone. The middle nasal concha is much smaller than the dorsal and ventral conchae and is located more caudally.

The following image comes from a website called WikiVet, it shows the nasal conchae within the nasal cavity:

Dissection of a Dog's Head Showing Nasal Conchae

 

Nasal Meatuses

The nasal meatuses are the spaces in between the nasal conchae that allow airflow. The dorsal nasal meatus provides airflow to the olfactory epithelium while the middle nasal meatus communicates with the paranasal sinuses. The ventral nasal meatus forms the main pathway for airflow to the pharynx. The common nasal meatus, which is located on either side of the septum, communicates with all the other nasal meatuses.

Vomeronasal Organ

The vomeronasal organ is a chemosensory organ and in mammals is located in the nasal cavity and is used for sensing mates and to recognise individuals. This organ is composed of two blind-ending ducts that lie within the hard palate and open rostrally to the incisive ducts which link the nasal and oral cavities. These ducts are lined with olfactory mucosa and are supported by cartilage. Mammals perform flehmen, the characteristic curl of the upper lip, when they are using the vomeronasal organ in order to suck in odours.

Guttural Pouch

The guttural pouches are large air-filled sacs that develop as a ventral diverticulum of the lower auditory tube in the horse. The pouches are found between the cranium and pharynx and are separated by a thin membranous median septum. They communicate with the nasopharynx through obliquely positioned slit-like clefts. The pterygoid muscles and parotid and mandibular glands cover the guttural pouches on their lateral sides. Ventrally they are split into lateral and medial components by the stylohyoid bone. They are clinically quite important because they are close to a number of important nerves and the carotid artery. Thus infections in this region can become serious.  

That's all for this post. As always, if you have any questions or comments please feel free to let me know :)