THORAX
Structures
- Pleura (serous membranes that cover the lungs and line the walls of the thorax)
 Visceral (pulmonary) pleura
 Parietal pleura (attached to thoracic wall via endothoracic fascia)
o Costal pleura
 Covers inner surfaces of the ribs and associated muscles)
o Diaphragmatic pleura
 Covers cranial surface of the diaphragm)
o Mediastinal pleura
 Left and right pleura form the lateral walls of the mediastinum and covers the lateral
surfaces of the structures within this space
 Pericardial mediastinal pleura
 Costodiaphragmatic recess
o Bilateral spaces formed from the reflection of the diaphragmatic pleura into costal pleura
o Accommodates the caudal borders of the lungs during inspiration
o Boundary for auscultation of lung sounds versus abdominal visceral sounds
- Mediastinum
 Cranial (cranial to the base of the heart)
 Middle (contains the pericardial sac)
 Caudal (caudal to the apex of the heart)
 Mediastinal recess (only on right side)
o Space b/w the right mediastinal pleura and plica vena cava
o Cavity completely occupied by the accessory lobe of the right lung
o Part of the pleural sac with NO connection to the mediastinum
- Plica vena cava
 Fold of the caudal mediastinal pleura only on the right side
 Contains the caudal vena cava and right phrenic n.
 Therefore, only the left phrenic n. is considered to be within the caudal mediastinum
- Thymus (within cranial mediastinum)
 Lymphoendothelial organ where T-lymphocytes development takes place
 Although replaced by fat in the adult dog, thymic remnants are commonly seen near the heart
Trachea
- Primary (principle) bronchi
 Left and right bifurcations of the trachea
- Carina
 Partition between the 2 primary bronchi at their origin from the trachea
 Important landmark for visualizing other associated structures on radiographs
- Lobar bronchi
 Further divisions of the primary bronchi that supply the lobes of the lungs
Lungs
- Visceral (pulmonary) pleura
- Pulmonary ligaments (left and right)
 Pulmonary pleura that has left the medial surfaces of the lungs to form a triangular membrane
continuous with the caudal mediastinal pleura and extends to the diaphragm
 On the medial surface of the caudal lobe of the left lung
 On the medial surface of the accessory and caudal lobes of the right lung
- Cupula of pleural sac
 The apex of each pleura sac
 Left pleural cupula extends more cranially beyond the first rib than the right pleural cupula
- Cardiac notch (pronounced on the right side)
 Fissure between the cranial and middle lung lobes
 Right ventricle of heart is accessible for cardiac auscultation and puncture (euthanasia) via this notch
- Left lung
 Cranial lobe (cranial and caudal parts)
 Caudal lobe
- Right lung
 Cranial lobe
 Middle lobe
 Accessory lobe (occupies the mediastinal recess of the right pleural sac)
 Caudal lobe
Heart
- Pericardium
 Fibrous pericardium
o Phrenicopericardial ligament
 Continuation of fibrous pericardium to the sternum and diaphragm
 Sternopericardial ligament (that part attaching to the sternum)
 Serous pericardium
o Parietal pericardium (lines fibrous pericardium)
o Visceral pericardium (epicardium that covers heart)
o Pericardial cavity
 Potential space between parietal and visceral serous pericardium
 Clinical importance?
- Surface anatomy
 Auricular surface
o Surface of heart facing the left thoracic wall; tips of auricles present
 Atrial surface
o Surface of heart facing right thoracic wall
 Coronary sulcus (contains coronary vessels and fat)
 Interventricular sulci
o Superficial separations of the left and right ventricles)
o Paraconal interventricular sulcus (auricular surface)
 Begins at base of pulmonary trunk
 Contains the paraconal interventricular branch of the left coronary a.
o Subsinuosal interventricular sulcus (atrial surface)
 Contains the subsinuosal interventricular branch of the left coronary a.
- Right atrium
 Sinus venarum
o Smooth portion of right atrium that incorporates the embryonic sinus venosus
o Separated from the right auricle by the crista terminalis
o Major veins draining into sinus venarum:
 Cranial vena cava
 Caudal vena cava
 Coronary sinus (terminal end of the great cardiac v.)
 Right azygous v. (sometimes)
 Interatrial septum
 Intervenous tubercle
o Tubercle located at confluence of incoming veins
o Directs inflowing blood from the cranial and caudal vena cava towards the right atrium
 Fossa ovalis
o Remnant of the foramen ovale, which shunted blood from right atrium to left (patent foramen
ovale results in interatrial septal defect (ASD))
 Right atrioventricular orifice
o Opening from right atrium to right ventricle
o Contains right AV valve
 Right auricle
o Pectinate muscles
o
-
-
-
 Help pump and direct blood flow into ventricles
Crista terminalis
 Semilunar crest that serves as junction b/w sinus venarum and right auricle
 Point of origin for pectinate muscles
 Conatins the SA node
Right ventricle
 Right AV valve
o Parietal cusp (arises from parietal wall)
o Septal cusp (arises from wall that houses septum)
o Sometimes has minor cusps
o Chordae tendinae
 Fibrous cords that connect valve cusps to papillar muscles of heart wall
 Prevent inversion of AV valve and backflow of blood into atrium
o Papillary muscles
 Conical muscular projections that serve as attachment points for chordae tendinae
 Prevent inversion of AV valve and backflow of blood into atrium
 Trabeculae carneae
o Muscular irregularities of ventricle wall
o Diminish blood turbulence
 Trabeculae septomarginalis
o Muscular band enxtending from septal to parietal walls of ventricle
o Contains purkinje fibers that helps to coordinate contraction of the ventricle
 Conus arteriosus
o Termination of the right ventricle that gives rise to the pulmonary trunk
 Pulmonary valve
o Consists of 3 semilunar cusps
 Pulmonary trunk
o Left pulmonary a.
o Right pulmonary a.
o Ligamentum arteriosum
 Remnant of ductus arteriosus, which shunted blood from pulmonary trunk to aorta
 Left recurrent laryngeal n. wraps around
Left atrium
 Left auricle
 Pectinate muscles
 Valve of foramen ovale (formed from septum primum)
 Left AV orifice
Left ventricle
 Left AV valve
o Pareital cusp
o Septal cusp
o Chordae tendinae
o Papillary muscles
 Trabeculae carneae
 Trabecular septomarginalis (sometimes)
 Aortic valve
o Consists of 3 semilunar cusps
 Aortic arch
o Left coronary a. (dominant blood supply in carnivores)
 Circumflex branch
o Subsinuosal interventricular branch
 Paraconal interventricular branch
o Right coronary a.
 Circumflex branch
Thoracic Vessels
- Arteries cranial to the heart
 Aorta (unpaired)
o Dorsal intercostal aa. (paired)
o Brachiocephalic trunk (unpaired)
 Common carotid aa.
o Subclavian aa. (paired)
 Vertebral a. (first branch to travel dorsally)
o Enters transverse foramen of C6, travels through transverse foramina of all
cervical vertebra, to supply the brain
 Costocervical trunk (second branch to travel dorsally)
o Branches become blood supply for first 3 dorsal intercostal aa.
 Superficial cervical a. (third branch to travel dorsally)
 Internal thoracic a. (opposite origin of superficial cervical a.)
- Veins cranial to the heart
 Cranial vena cava (unpaired; union of brachiocephalic veins)
o Brachiocephalic vv.
 Union of external jugular and subclavian vv.
 External jugular vv. (main return of blood supply from the head)
 Subclavian vv.
 Azygos v. (unpaired)
o Passes through aortic hiatus
o Empties into right atrium at the termination of the cranial vena cava
- Intercostal arteries and veins
 Dorsal intercostal aa. and vv.
o Dorsal branches (supply epaxial mm.)
o Ventral branches
 Supply the intercostal mm.
 Run along the caudal border of each rib
o Lateral cutaneous branches
 Supply cutaneous structures like the thoracic mammary glands
 Internal thoracic a. and v.
o Surgical important of internal thoracic vessels?
o Ventral intercostal branches
 Anastomose with the ventral branches of the dorsal intercostal aa. and vv.
o Perforating branches (supply cutaneous structures)
o Cranial epigastric a.
 Terminal branch of the internal thoracic a.
 Passes caudally on the deep surface of the rectus abdominis m.
 Cranial superficial epigastric a
o Supplies skin over the rectus abdominis m.
o Supplies the caudal thoracic and cranial abdominal mammary glands
Innervation
- Thoracic spinal nn. (emerge from intervertebral foramen)
 Dorsal branches
 Intercostal nn. (ventral branches of the first 12 thoracic spinal nerves)
o Lateral cutaneous branches
o Ventral cutaneous branches
- Lateral thoracic n.
 Emerges from the axilla between the latissimus dorsi and deep pectoral mm. to terminate on the ventral
border of the cutaneous trunci m.
 GSE to the cutaneous trunci m.
-
Phrenic nn. (paired)
 Arise from ventral branches of the 5th, 6th, and sometimes 7th cranial nerves
 The phrenic nerves contain motor, sensory, and sympathetic nerve fibers
 Provide the only GSE to the diaphragm as well as GSA to the central tendon
 Left phrenic n.
o Within the caudal mediastinum
 Right phrenic n.
o NOT within the caudal mediastinum b/c within the plica vena cava
- Sympathetic trunk (paired)
 Strand of nerve fibers from the sympathetic division of the ANS
- Sympathetic chain ganglion
 Contain cell bodies of postganglionic sympathetic neurons
 Usually located in the sympathetic trunk at the point where each rami communicans joins it
- Rami communicans
 Nerve fiber branches that join (communicate) spinal nerves and sympathetic trunk ganglia
 Each preganglionic sympathetic axon must pass through here to reach the sympathetic trunk, though
not all synapse within chain ganglia (splanchnic nn., head nn. that synapse at cranial cervical ganglion)
 Rami communicans of spinal cord segments T1-L5 contain both preganglionic and postganglionic axons
- Sympathetic ganglia of cranial mediastinum
 Cervicothoracic ganglion (paired)
o Formed from fusion of caudal cervical ganglion and first 2-3 thoracic ganglia
o Rami communicans leave this ganglion and connect to the ventral branches of spinal nerves C7,
C8, T1 and T2, which help form the brachial plexus, a pathway for postganglionic axons to
reach the thoracic limb
o Vertebral n.
 Branch that follows the vertebral a. through the cervical vertebra transverse foramina
o Ansa subclavia
 Middle cervical ganglion (paired)
o Junction of the ansa subclavian and vagosympathetic trunk
o Cardiac nn.
 Branches from the ansa subclavian and middle cervical ganglion go to the heart
 Both sympathetic (pre- and postganglionic) and parasympathetic (preganglionic) fibers
- Vagosympathetic trunk
 Joining of the sympathetic trunk and vagus nerve just cranial to the middle cervical ganglion
 SNS portion carries pre- and postganglionic fibers cranially to structures in the head
 PNS portion (vagus n.) contains preganglionic fibers that course caudally to thoracic/abdominal organs
- Vagus n.
 Left recurrent laryngeal n. (longer, faster nerve conduction velocity)
o Wraps around the ligamentum arteriosum (patent ductus arteriosis)
 Right recurrent laryngeal n. (shorter)
o Wraps around the right subclavian a.
 Dorsal vagal trunk (unites more caudally)
o Joining of dorsal branches of left and right vagus n.
 Ventral vagal trunk
o Joining of ventral branches of left and right vagus n.
Lymph nodes
- Thoracic duct
- Tracheobronchial lymph nodes (always present)
 Left/Right
o Located on the lateral surfaces of their respective primary bronchi
 Middle (largest)
o Located at the angle of bifurcation of the trachea
- Cranial sternal lymph node (paired or unpaired)
 Found near midline on sternum cranial to transversus thoracis m.
-
Cranial mediastinal lymph nodes (variable number)
 Found in cranial mediastinum along large vessels of the heart