Anatomy of the Mouth

Hello :) In this post we'll take a look at the anatomy of the mouth. This includes the anatomy of the tongue's papillae, the internal structures of the tongue, the muscles of mastication, swallowing, the innervation of the mouth as well as salivary glands. Enjoy!

Tongue Papillae

There are five types of keratinised papillae that can be distinguished on an animal’s tongue.

• Filiform: These are fine and thread-like and are found on the tip of the tongue. They assist in the movement of food. 
•  Dogs have many of these 
• They are the most common type of papillae in horses. They aid in grasping and steering the food caudally 
• They are also numerous in cattle where they are cornified and directed caudally.
• Fungiform: these are taste buds which detect sweet and salty tastes and lie mainly laterally 
• Valate: these are round and located in pits on the caudal area of the dorsal surface of the tongue. They have secondary tastebuds which detect sour tastes. 
•  Dogs have two pairs of these. 
• Foliate: these are also round but are not located in pits. They can be found rostral to the valate papillae and have secondary tastebuds which detect sour tastes. 
• Conical: these are coarse and pointed papillae that are found on the dorsal surface of the tongue. 
•  In dogs they are numerous at the base of the epiglottis.

Interestingly, in cattle a prominent protuberance called the torus linguae is present in the middle of the dorsal surface of the tongue. Numerous flattened papillae, called lentiform or lenticular papillae are present on the surface of this. It allows the cow to generate high pressures with its tongue during rumination in order to extract as much fluid as it can from its cud.

Internal Structures of the Tongue

Muscles

Four pairs of extrinsic muscles which help to move the tongue exist. The styloglossus originates from the stylohyoid and retracts and elevates the base of the tongue. The hyoglossus comes from the basihyoid and retracts the tongue. The genioglossus come from the inscisive region of the mandible. Its rostral elements retract the tongue while its distal elements pull it forward. The myolohyoids form a sling which supports the ventral surface of the tongue. 

Blood Vessels

The lingual and sublingual branches of the linguofacial trunk (which arises from the external carotid) provides the arterial blood supply. 

The deep lingual and sublingual veins drain to the lingual which drains to the linguofacial which drains into the maxillary vein. 

The superficial and deep lymph vessels drain caudally to the retropharyngeal lymph nodes.

Muscles of Mastication

 

Temporalis

Originates from an extensive area on the lateral surface of the cranium and inserts on the coronoid process of the mandible. This works to move the jaw upwards when it contracts and is well developed in animals in which the primary jaw movement is scissor-like (eg. The dog and cat)

Masseter

This lies lateral to the mandible and originates at the maxillary region of the skull and zygomatic arch and inserts on the more caudal part of the mandible. Frequently multipennate. The general effect of contraction is to raise the mandible and draw it to the active side. It is better developed in herbivores as they make lateral and rotational movements when chewing. 

Pterygoid

This lies medial to the mandible and passes to the mandible through the pterygopalatine region of the skull. Its function is to raise the mandible and draw it inward with some protrusion. Well developed in herbivores. 

Digastricus

This passes from the an area on the skull caudal to the temperomandibular joint to the ventral margin of the mandible. It functions to open the mouth.

Swallowing

Before we discuss the mechanisms behind swallowing, let’s first take a look at the anatomy of the pharyngeal region. 

Pharynx

The pharynx is a musculomembranous tube caudal to the oral and nasal cavities. It has three parts: oropharynx, laryngopharynx and nasopharynx. The pharynx has two entrances, the oral and nasal cavities, and one exit, the oesophagus. 

The oropharynx extends from the last cheek tooth to the epiglottis and sits inside the dental arcade. The soft palate covers it dorsally and the tongue encloses it ventrally. The oropharynx is separate to the oral cavity as it is narrower, and acts involuntarily when swallowing, it is also innervated by the glossopharyngeal nerve (the oral cavity is innervated by the trigeminal). The pharyngeal epithelium is also derived from the foregut endoderm while the oral cavity epithelium is from the stomodeal ectoderm. 

The nasopharynx extends from the internal nares to a transverse plane through the caudal edge of the soft palate. It lies between the hard and soft palates.

The laryngopharynx extends from a transverse plane through the apices of the arytenoid cartilage to the level of the cricoid cartilage. The pharynx is a crossover area of the alimentary tract and respiratory pathways.

Pharyngeal Muscles

These basically form strips of muscle which travel dorso-ventrally across the pharynx and serve to constrict the pharynx. There are five muscles which form these strips. From rostral to caudal, they are: palatopharyngeus and pterygopharyngeus, these are termed the rostral constrictors; hyopharyngeus, the middle contrictor; thyropharyngeus, and cricopharyngeus, these are the caudal constrictors. An oesophageal sphincter is located at the rostral limit of the oesophagus.  The pharyngeal branch of the vagus nerve innervates these muscles. 

Soft Palate

This extends from the hard palate to the epiglottis and closes of the nasal chambers to prevent food entering them. It also closes the nasopharynx to allow a vacuum to be created during suckling. Normally it lies in contact with the ventral edge of the epiglottis but it is lifted dorsally during swallowing, vomiting and mouth breathing. A palatoglossal arch extends from the root of the tongue up to the middle of the lateral attachments of the soft palate. However, dogs don’t have a true palatoglossal arch.

Epiglottis

This is a leaf-like membrane-covered flexible cartilage that lies at the entrance to the larynx. Its ventral surface is attached to the base of the tongue, basihyoid and thyroid cartilage. The epiglottis is joined to the arytenoid cartilages by the aryepiglottic folds.

Now that we have some knowledge about the anatomy of the pharyngeal region, lets look at how swallowing (aka deglutition) works. 

Swallowing

This is initiated voluntarily but completed involuntarily.

• 1.       A bolus is formed at the back of the tongue. 
• 2.       The bolus is then pressed against the hard palate and squeezed caudally. 
• 3.       The bolus enters the oropharynx. The soft palate is elevated to close off the nasal chamber, the tongue is jerked caudally by the styloglossus and this propels the bolus further caudally. 
• 4.       The hyoid apparatus is swung rostrodorsally by the geniohyoid muscle which brings the pharynx to meet the bolus. This movement also swings the larynx forwards which pushes the epiglottis back to close the entry to the larynx. 
• 5.       The pharyngeal constrictor muscles relax 
• 6.       They then contract sequentially to propel the bolus caudally 
• 7.       The tongue relaxes and returns to its normal position 
• 8.       The cranial oesophageal sphincter relaxes 
• 9.       The bolus is propelled down the oesophagus by the momentum imparted from the pharynx, the contraction of the oesophageal walls and gravity 
• 10.   The hyoid apparatus returns to its resting position and the laryngeal airway reopens to allow breathing. Resting tone then returns to the pharynx.

Innervation of the Mouth

The rostral two thirds of the tongue receive general sensory innervation from the lingual branch of the mandibular nerve. The chorda tympani (a branch of the facial nerve) allows the sensation of taste in this area. The glossopharyngeal nerve provides both special and general sensory innervation to the caudal third of the tongue. The extrinsic and intrinsic muscles are supplied by the hypoglossal nerve.

Salivary Glands

Parotid

The parotid glad is a diffuse lobulated serous gland that lies ventral to the ear mainly in the space between the mandibular ramus and the wing of the atlas. 

On its lateral surface, it is covered by skin, fascia and some cutaneous muscles (the parotidoauricularis). The external jugular vein runs superficially and obliquely along its ventral border. Its medial surface lies adjacent to the stylohyoid bone, the masseter and digastric muscles, tendons of the brachiocephalic and sternocephalic muscles and the external carotid artery. The facial nerve passes through the gland itself. The parotid duct crosses the masseter obliquely and enters the cheek opposite P4. 

Mandibular

This is a large, rounded and curved discrete gland which extends from a fossa in the atlas to the basihyoid bone and is partly covered by parotid gland. Its duct runs on the medial surface of the mandible and opens on the floor of the oral cavity opposite the canine tooth. 

Sublingual

This lies beneath the mucous membrane between the body of the tongue and the mandibular ramus. It can be mono- or polystomatic in all species except the horse which only has monostomatic. Mixed secretion which lubricates the mouth.

Zygomatic

Aka, orbital gland

This is found only in carnivores and lies ventral to the zygomatic arch. It is encapsulated and covered in fat.

That's it for this post, if you have any questions please feel free to ask :)