Comprehensive Notes on the Neck, Thorax, and Thoracic Limb of the Dog

Internal Thoracic Artery and Associated Structures

• The internal thoracic artery gives off several branches to surrounding structures:

  • The phrenic nerve.

  • The thymus.

  • The mediastinal pleurae.

  • The dorsal intercostal spaces.

• Perforating branches are distributed to the superficial structures of the ventral third of the thorax.

• Anastomoses occur with the dorsal intercostal arteries on the medial side of the thoracic wall.

• Termination of the internal thoracic artery: Occurs near the attachment of the costal arch with the sternum.

  • Musculophrenic artery: Runs caudodorsally in the angle formed by the diaphragm and lateral thoracic wall.

  • Cranial epigastric artery: The larger of the two terminal branches, which includes a cranial superficial epigastric branch.

• Dissection note: The mediastinum should be cut near the sternum and reflected cranially to observe these structures.

Anatomy of the Lungs

• Lobe Division: Each lung is divided into lobes based on the branching pattern of its principal bronchus into lobar bronchi.

• Left Lung Characteristics:

  • Divided into cranial and caudal lobes by deep fissures.

  • The cranial lobe is further divided into cranial and caudal parts.

• Right Lung Characteristics:

  • Divided into cranial, middle, caudal, and accessory lobes.

  • Accessory Lobe: Can be observed from the left through the caudal mediastinum or from the right through the plica venae cavae, situated between these structures.

• Cardiac Notch: Located on the right lung at the $4th$ and $5th$ intercostal spaces.

  • The apex of the notch is continuous with the fissure between the cranial and middle lobes.

  • A larger area of the ventral convexity of the heart is exposed on the right side.

  • The right ventricle occupies this area and is the site for cardiac puncture.

• Lung Hilus: The point where structures enter the lung. Removal involves transecting structures far enough from the heart to protect vagal nerves but close enough to keep lobes together.

• Structural Impressions:

  • Left Lung: Displays a long aortic impression.

  • Right Lung: The accessory lobe has marked impressions from the caudal vena cava on one side and the esophagus on the other.

  • General: Vascular impressions on cranial lobes and costal impressions on each lung.

The Bronchial Tree

• Trachea Bifurcation: The trachea divides into left and right principal bronchi.

• Carina: The partition at the origin of the principal bronchi from the trachea.

• Bronchial Ring Structure: Identified by the cartilage rings within their walls.

• Branching Pattern of the Right Principal Bronchus:

  • Right Cranial Bronchus: Arises at the level of the carina, directed laterally to the right cranial lobe.

  • Right Middle Bronchus: Branches off ventrally.

  • Terminal Branches: Accessories bronchus (medial) and right caudal bronchus (lateral).

• Branching Pattern of the Left Principal Bronchus:

  • Left Cranial Bronchus: Arises laterally and divides into cranial and caudal parts.

  • Terminal Branch: Left caudal bronchus (continues caudally).

• Segmental Bronchi: Branch off lobar bronchi dorsally and ventrally, except in the right middle lobe where they are cranial and caudal.

Vascular and Lymphatic Systems of the Thorax

• Pulmonary Veins: Usually a single vein from each lobe drains directly into the left atrium of the heart.

  • Note on Specimen Preparation: Red latex in pulmonary veins is due to retrograde injection through the carotid artery ($carotid \rightarrow aorta \rightarrow left \ ventricle \rightarrow left \ atrium \rightarrow pulmonary \ veins$).

  • Latex usually does not cross capillary beds into pulmonary arteries unless pressure causes an interatrial or interventricular septum rupture.

• Pulmonary Trunk: Supplies each lung with a pulmonary artery.

  • Left Pulmonary Artery: Usually cranial to the left principal bronchus at the lung root.

  • Right Pulmonary Artery: Ventral to the right principal bronchus.

• Tracheobronchial Lymph Nodes: Located at the tracheal bifurcation and further out on the bronchi.

• Cranial Vena Cava: Formed by the union of right and left brachiocephalic veins at the thoracic inlet; drains into the right atrium.

  • Brachiocephalic Vein: Formed on each side by the external jugular and subclavian veins.

• Azygos Vein: Only the right azygos vein develops in the dog. It enters the cranial vena cava (last branch) or the right atrium directly. It collects dorsal intercostal veins as far cranially as the $3rd$ or $4th$ intercostal space.

• Thoracic Duct: Chief lymph channel.

  • Origin: Begins as a continuation of the cisterna chyli in the sublumbar region between the diaphragmatic crura.

  • Course: Runs cranially on the right dorsal border of the thoracic aorta and ventral border of the azygos vein to the $6th$ thoracic vertebra. It crosses the $5th$ thoracic vertebra to the left side and continues to the left brachiocephalic vein.

  • Function: Receives lymph from abdominal/pelvic viscera, pelvic limbs, left thoracic limb, and left tracheal trunk.

• Right Lymphatic Duct: Receives lymph from the right thoracic limb and right tracheal trunk; enters near the right brachiocephalic vein.

Arteries Cranial to the Heart

• Aorta Structure:

  • Ascending Aorta: Emerges from the left ventricle, covered by pericardium.

  • Aortic Arch: Sharp bend dorsally and to the left.

  • Descending Aorta: Runs caudally ventral to the vertebrae (Thoracic aorta cranial to diaphragm; Abdominal aorta caudal to it).

• Coronary Arteries: Right and left branches of the ascending aorta supplying heart muscle.

• Brachiocephalic Trunk: First branch of the aortic arch. Gives rise to the left common carotid artery and terminates as the right common carotid and right subclavian arteries.

• Left Subclavian Artery: Originates from the aortic arch beyond the brachiocephalic trunk.

• Branches of the Right Subclavian Artery (arising medial to the $1st$ rib):

  • 1. Vertebral Artery: Crosses the medial surface of the $1st$ rib, enters the transverse foramen of the $6th$ cervical vertebra, and passes through the first $6$ cervical vertebrae to the vertebral canal.

  • 2. Costocervical Trunk: Supplies the first $3$ intercostal spaces and muscles of the neck and thoracic vertebrae.

  • 3. Superficial Cervical Artery: Emerges from the thoracic inlet to supply the neck base and scapular region.

  • 4. Internal Thoracic Artery.

• Thoracic Aorta Branches:

  • Bronchoesophageal Artery: Usually leaves the right $5th$ intercostal artery to supply the esophagus and lungs (via bronchial arteries).

  • Dorsal Intercostal Arteries: $8$ to $9$ pairs, starting at the $4th$ or $5th$ intercostal artery and continuing caudally.

The Brachial Plexus and Thoracic Limb Arteries

• Brachial Plexus Formation: Ventral branches of the $6th$, $7th$, and $8th$ cervical ($C-6, C-7, C-8$) and the $1st$ and $2nd$ thoracic ($T-1, T-2$) spinal nerves.

  • Named Nerves: Suprascapular, subscapular, axillary, musculocutaneous, radial, median, ulnar, thoracodorsal, lateral thoracic, and pectoral nerves.

• Main Arterial Path of the Thoracic Limb:

  • Subclavian Artery: From origin to the $1st$ rib.

  • Axillary Artery: From the $1st$ rib to the conjoined tendons of the teres major and latissimus dorsi.

  • Brachial Artery: From the teres major/latissimus dorsi to the terminal median artery distal to the elbow.

• Superficial Cervical Lymph Nodes: Usually $2$ nodes lying on the serratus ventralis and scalenus, receiving lymph from the neck, head, and thoracic limb.

Detailed Branches of the Axillary Artery

• 1. External Thoracic Artery: Distributed to the superficial pectorals.

• 2. Lateral Thoracic Artery: Runs caudally along the deep pectoral; supplies latissimus dorsi, deep pectoral, cutaneous trunci, and thoracic mammae.

• 3. Subscapular Artery: Larger than the continuation of the axillary. It enters the nutrient foramen of the scapula and has two major branches:

  • Thoracodorsal Artery: Supplies the teres major, latissimus dorsi, and skin.

  • Caudal Circumflex Humeral Artery: Passes between the humerus and teres major; supplies the triceps, deltoideus, coracobrachialis, infraspinatus, and shoulder joint capsule.

• 4. Cranial Circumflex Humeral Artery: Vessel arising from the axillary artery distal to the subscapular artery.