The Lower Respiratory Tract

Hi :) In this post we'll take a look at the anatomy of the structures within the lower respiratory tract. This includes the trachea, lungs, pleura and diaphragm. 



Trachea

The trachea extends from the cricoid cartilage of the larynx to the tracheal bifurcation. It is supported by C-shaped rings of cartilage which prevent the trachea from collapsing and also allow the dilation of the oesophagus during deglutition. The rings open dorsally and the gap in the “c” shape is closed by the tracheal muscle. The rings of cartilage are connected by bands of fibroelastic tissue.

The trachea is lined by respiratory mucosa which is composed of pseudostratified ciliated epithelium. The cilia continually drive mucous and debris cranially towards the pharynx. 

Within the neck, the external aspect of the trachea is lined with loose connective tissue known as adventitia and this connects the trachea to surrounding structures. The caudal (recurrent) laryngeal nerve passes within this adventitia. The trachea passes ventral to the cervical spine and longus colli muscle until it reaches the thoracic inlet. This part of the trachea maintains a median position in relation to the oesophagus. Ventrally, it is connected to the long hyoid muscles, while the common carotid artery and vagosympathetic trunk pass on its lateral sides. 

Within the mediastinum of the thorax, the trachea deflects to the right where it crosses the aortic arch. Dorsal to this part of the trachea is the cranial vena cava, aortic arteries and oesophagus. The mediastinal lymph nodes and thymus (in young animals) lie ventrally.

At the fifth intercostal space, the trachea bifurcates to give off the primary bronchi. Three primary bronchi branch on the right hand side, while two branch in the left. This forms the pattern for the lobes of the lung.

The Lungs

Form

The lungs can be divided into lobes. In the dog, the right lung has cranial, middle, caudal and accessory lobes while the left lung has cranial and caudal lobes. However, in the left lung there are two portions to the cranial lobe: the cranial portion of the cranial lobe and the caudal portion of the cranial lobe.

There are differences in form between species, with the pig and ruminants having an extra branch of the trachea known as the tracheal bronchus. This branches to the right above the tracheal bifurcation and opens into the cranial lobe of the right lung. In addition, carnivores have deeper fissures in between their lobes while in species such as the horse and cow these fissures are quite shallow. This is due mainly to the flexibilty of the thorax in these animals.

Blood Supply

The pulmonary arteries enter the lungs at the tracheal bifurcation and generally follow the bronchi through the lungs. The pulmonary veins don’t always follow the bronchial tree and may run separately. The pulmonary arteries carry deoxygenated blood to the lungs while the pulmonary arteries carry oxygenated blood back to the heart. The lungs receive a nutritional blood supply from the broncho-oesophageal artery and vein.

Lymph Nodes

Lymph drains into the tracheobronchial lymph nodes which drains into the mediastinal lymph nodes which empty into the thoracic duct.

Innervation

Parasympathetic and sympathetic fibres innervate the lungs and are derived from the pulmonary plexus within the mediastinum. Efferent fibres supply the bronchial glands and muscles as well as the blood vessels. Afferent fibres originate from the mucosa and stretch receptors.  

Pleura

There are two pleural cavities in the thoracic cavity which are located on the right and left of the mediastinum. These cavities are lined with pleura which is a serous membrane. The pleura extends from the thoracic wall to cover the lungs and also lines the diaphragm and mediastinum. There are two main sections of pleura: parietal and visceral. The visceral pleura surround the lungs and may be called the pulmonary pleura. The parietal pleura surround the other structures, it includes costal pleura (which lines area formed by the ribs), the mediastinal pleura (covering the mediastinum), and the diaphragmatic pleura (lining the diaphragm).

Between the parietal and visceral pleura is a narrow space filled with serous fluid. This space has a negative pressure which keeps the lungs “stuck” to the walls of the thorax.

Diaphragm

The diaphragm separates the thoracic and abdominal cavities and is dome shaped. It has a triangle-shaped central tendon and a muscular periphery. The periphery includes the left and right crura which are connected to the ventral aspect of the first three or four lumbar vertebrae by tendons. The right crus is larger than the left crus. The costal part of the muscular periphery is much thinner.

The diaphragm has three main openings: the aortic, oesophageal hiatuses and the caval foramen. The aortic hiatus is the most dorsal and the aorta, azygous vein and thoracic duct pass through it. The oesophageal hiatus lies more ventrally and allows the oesophagus, the dorsal and ventral vagal trunks that accompany it and the vessels that supply the oesophagus into the abdominal cavity. The caval foramen lies within the central tendon and provides an entry point for the caudal vena cava into the abdominal cavity. 

Hyoid Apparatus

Hi :) This post is about the hyoid apparatus and will be quite short. It deals with quite an important structure though, as the hyoid apparatus functions in holding the larynx in place and supports the pharynx and tongue. 

The hyoid apparatus contains a number of interconnected bones and cartilages that suspend the larynx and the base of the tongue. 

Bones

The basihyoid is the only transverse bone and connects the left and right sides of the apparatus. In carnivores it is simple but in herbivores, which tend to have large tongues, the basihyoid may have a rostral lingual process. This supports the root of the tongue.

The thyrohyoids connect the basihyoid with the rostral corner of the thyroid cartilage.

The ceratohyoids run rostrally from the basihyoids to the epihyoids, lateral to the base of the oropharynx and tongue.

The epihyoids run vertically between the ceratohyoids and stylohyoids.

The stylohyoids connect the epihyoids with the skull through the tympanohyoid cartilages. 

The following image shows how these bones are arranged in the dog:

Canine Hyoid
Source

 And that's all for the hyoid apparatus :)

 

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 :)