Lab Instructions:

The limited dissection required is performed by teams on carnivore cadavers.  Otherwise much of the content is also learned on available lab prosections and models, wet and dry.  Once the carnivore material has been considered, move to the ungulate specimens to learn comparative features and to complete the learning goals.

Lungs

The right and left lungs are semi-pyramidal shaped organs possessing a rounded cranial end, called the apex, and a slightly concave, and much broader, caudal end, called the base. Each lung is divided into lobes based on the branching pattern of its principal bronchus into lobar bronchi (bronchi will be described in a later section). The left lung is divided into cranial and caudal lobes by deep fissures. The left cranial lobe is further divided into cranial and caudal parts. The right lung is divided into cranial, middle, caudal, and accessory lobes. The accessory lobe is tucked ventromedially into a deep space called the mediastinal recess, bounded by the plica vena cava and caudal vena cava laterally, and the mediastinum medially.  Therefore, the accessory lobe can be viewed from the left pleural cavity by looking through the relatively thin, translucent caudal mediastinum.

Each lung has a root and a hilus.  The root of the lung (radix pulmonis) refers to the structures that enter and leave the lung, i.e. vessels, bronchi, lymphatics.  Recall, these structures are covered by pleura as it continues from the mediastinal (parietal) pleura onto the lung surface to become pulmonary (visceral) pleura.  The root of the lung attaches the lung to the mediastinum.  The hilus of the lung (hilux pulmonis) is the area on the lung where the structures of the root enter and leave.

The cardiac notch (already studied in the Cardiovascular System) is a lung border feature (i.e. it is not a heart feature, despite its name), and it is prominent on the right lung in carnivores.  It is identified as the wide, ‘triangular indentation’ in the lung border as it passes from the cranial to the middle lung lobes.  The apex of the notch is continued by the interlobar fissure between the cranial and middle lobes.  Because of the cardiac notch (and hence it being so named), a large area of the right aspect of the heart wall (the region of the right ventricle) is visible, and in direct proximity to the thoracic wall (see clinical relevance box).

Trachea and bronchi (singular, bronchus)

Tracheal anatomy was also considered in Lab 2A.

The trachea (the ‘windpipe’) consists of a series of connected tracheal cartilages – C-shaped cartilaginous structures, positioned with their opening dorsally. This dorsal space is filled with transversely aligned tracheal muscle, joining the two ends of the cartilage. In carnivores the tracheal muscle is on the outside of the cartilages, in other species it is located on the inside of the cartilages.  Annular ligaments bind one tracheal cartilage to the next, and a mucous membrane and submucosa line the lumen.  The trachea is subdivided into a cervical part and a thoracic part.

At the tracheal bifurcation the left and right principal bronchus (bronchus principalis) arise. The carina is the luminal vertical partition or crest, between the principle bronchi at their origin from the trachea.  The carina is viewed during bronchoscopy.  Each principal bronchus divides into lobar bronchi (bronchi lobares), providing the conduit for air to enter and leave individual lung lobes.  There is one lobar bronchus per lobe of lung, therefore in the carnivore, there are four lobar bronchi on the right, and two on the left.  On the left the cranial lobar bronchus quickly divides into cranial and caudal parts to supply the same named parts of that lung lobe. Cartilaginous bronchial rings extend into the lobar bronchi. Within each lung lobe, segmental bronchi (bronchi segmentales) branch off the lobar bronchus dorsally and ventrally except for the right middle lobe, where they are positioned cranial and caudal.

Recall from the Cardiovascular System, that the pulmonary trunk supplies each lung with a pulmonary artery. At the root of the lung, the left pulmonary artery usually lies cranial to the left principal bronchus. The right pulmonary artery is ventral to the right principal bronchus. The artery and bronchus are at a more dorsal level than the veins (recall this detail when considering radiographs of the thorax – ‘veins are ventral and central’ is a catchy way to remember pulmonary veins are ventral to the bronchus and pulmonary artery on lateral radiographs and medial to those same structures on dorsoventral and ventrodorsal radiographs).

Comparative ungulate anatomy

We know now that the typical lung lobation pattern is 2 lobes on the left and 4 on the right. And important to remember, lung lobation is based on the branching pattern of the bronchial tree, and not on the presence or absence of obvious fissures between regions of the lung.

As for carnivores, the C-shaped tracheal cartilages are positioned with their opening dorsally. Distinct from carnivores, the trachealis m. is attached on the inside/deep surface of the tracheal cartilages in ungulates.  Annular ligaments bind one tracheal cartilage to the next, and a mucous membrane and submucosa line the lumen.  The trachea is subdivided into a cervical part and a thoracic part.  The cross-sectional shape of the trachea varies between species.

Horse notes

With the exception of the accessory lobe, the lungs of the horse are not divided into distinct lobes by deep fissures and the horse has no right middle lung lobe. The cardiac notch is present, left and right, and partially separates each equine lung into relatively small cranial and very large caudal lobes.

The horse trachea is flattened dorsoventrally and consists of 48-60 cartilages. The tracheal bifurcation is clearly visible on endoscopic examination at the site of the carina.

Artiodactyl notes

The lungs of ruminants and the pig do have distinct lobes and, in ruminants, both cranial lobes are subdivided into cranial and caudal parts, whereas in the pig, only the left cranial lobe is so divided (similar to the dog and cat)   A cardiac notch is present on the left and right sides.  Intralobar lobulation of the lung is clearly evident in the ox, less conspicuous in the goat and the pig, and hardly detectable in the sheep.

The artiodactyl trachea has a distinct feature not seen in the carnivore or horse.  The right cranial lung lobe receives a dedicated tracheal bronchus (bronchus trachealis). This bronchus exits the trachea a notable distance cranial to the tracheal bifurcation into the standard left and right principal bronchus.  Awareness of the tracheal bronchus in our ruminant and pig patients is highly relevant, for example, when endotracheal intubation is performed.

The ruminant trachea consists of 48-60 cartilages.  It is relatively oval in cross section on the live ox, whereas in the goat it is more U-shaped on cross section.  The sheep has 3 regions of different cross sectional shape along the length of its trachea.  As already stated, the tracheal bronchus specifically supplies the right cranial lung lobe (cranial and caudal parts).

The pig trachea is circular in cross section and has a range of 32-36 cartilages. The ends of the incomplete cartilaginous rings overlap each other.  As already stated, the tracheal bronchus specifically supplies the right cranial lung lobe.

Lung auscultation – horse, ruminant, pig

In the squarely standing ox, horse and pig, the olecranon is at about the level of the fifth costochondral junction. This is a useful clinical landmark for underlying thoracic structures.

Recall, the basal border of the lung is the caudoventral margin of the lungs, primarily traced along the caudal edge of the caudal lung lobe.  This basal border location can be approximated on the external thorax to provide a guide as to the caudoventral extent of the lung (keeping in mind the border moves with respiration). In the horse this border passes from the 6th costochondral junction in a sweeping curve through the middle of the 11th rib and ends at the vertebral end of the 16th rib (at the epaxial muscle margin). In the ruminant, this border is steeper, starting again at the 6th costochondral junction, passing through the middle of the 10th rib, and ending at the vertebral end of the 11th intercostal space (i.e. the space caudal to the 11th rib).

For each species then, the surface area for lung auscultation is defined as: 1) cranially, the caudal border of the long head of the triceps muscle, 2) dorsally, the lateral margin of the epaxial mm. and 3) caudoventrally, the basal border of the lung (clinically, this boundary may be approximated by a line drawn from the dorsal last rib to the point of the elbow i.e. the olecranon).  It is worth having these boundaries in mind when listening to the chest of a horse or ruminant or pig, to be sure we cover the entire lung area!