Thorax structures



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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)

      • Costal pleura

        • Covers inner surfaces of the ribs and associated muscles)

      • Diaphragmatic pleura

        • Covers cranial surface of the diaphragm)

      • 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

      • Bilateral spaces formed from the reflection of the diaphragmatic pleura into costal pleura

      • Accommodates the caudal borders of the lungs during inspiration

      • 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)

      • Space b/w the right mediastinal pleura and plica vena cava

      • Cavity completely occupied by the accessory lobe of the right lung

      • 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

      • Phrenicopericardial ligament

        • Continuation of fibrous pericardium to the sternum and diaphragm

        • Sternopericardial ligament (that part attaching to the sternum)

    • Serous pericardium

      • Parietal pericardium (lines fibrous pericardium)

      • Visceral pericardium (epicardium that covers heart)

      • Pericardial cavity

        • Potential space between parietal and visceral serous pericardium

        • Clinical importance?

  • Surface anatomy

    • Auricular surface

      • Surface of heart facing the left thoracic wall; tips of auricles present

    • Atrial surface

      • Surface of heart facing right thoracic wall

    • Coronary sulcus (contains coronary vessels and fat)

    • Interventricular sulci

      • Superficial separations of the left and right ventricles)

      • Paraconal interventricular sulcus (auricular surface)

      • Subsinuosal interventricular sulcus (atrial surface)

        • Contains the subsinuosal interventricular branch of the left coronary a.

  • Right atrium

    • Sinus venarum

      • Smooth portion of right atrium that incorporates the embryonic sinus venosus

      • Separated from the right auricle by the crista terminalis

      • 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

    • Fossa ovalis

      • 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

      • Opening from right atrium to right ventricle

      • Contains right AV valve

    • Right auricle

      • Pectinate muscles

        • 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

      • Parietal cusp (arises from parietal wall)

      • Septal cusp (arises from wall that houses septum)

      • Sometimes has minor cusps

      • 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

      • 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

      • Muscular irregularities of ventricle wall

      • Diminish blood turbulence

    • Trabeculae septomarginalis

      • Muscular band enxtending from septal to parietal walls of ventricle

      • Contains purkinje fibers that helps to coordinate contraction of the ventricle

    • Conus arteriosus

    • Pulmonary valve

      • Consists of 3 semilunar cusps

    • Pulmonary trunk

      • Left pulmonary a.

      • Right pulmonary a.

      • 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

      • Pareital cusp

      • Septal cusp

      • Chordae tendinae

      • Papillary muscles

    • Trabeculae carneae

    • Trabecular septomarginalis (sometimes)

    • Aortic valve

      • Consists of 3 semilunar cusps

    • Aortic arch

      • Left coronary a. (dominant blood supply in carnivores)

        • Circumflex branch

          • Subsinuosal interventricular branch

        • Paraconal interventricular branch

      • Right coronary a.

        • Circumflex branch

Thoracic Vessels

  • Arteries cranial to the heart

    • Aorta (unpaired)

      • Dorsal intercostal aa. (paired)

      • Brachiocephalic trunk (unpaired)

        • Common carotid aa.

      • Subclavian aa. (paired)

        • Vertebral a. (first branch to travel dorsally)

          • Enters transverse foramen of C6, travels through transverse foramina of all cervical vertebra, to supply the brain

        • Costocervical trunk (second branch to travel dorsally)

          • 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)

      • 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)

      • Passes through aortic hiatus

      • Empties into right atrium at the termination of the cranial vena cava

  • Intercostal arteries and veins

    • Dorsal intercostal aa. and vv.

      • Dorsal branches (supply epaxial mm.)

      • Ventral branches

        • Supply the intercostal mm.

        • Run along the caudal border of each rib

      • Lateral cutaneous branches

        • Supply cutaneous structures like the thoracic mammary glands

    • Internal thoracic a. and v.

      • Surgical important of internal thoracic vessels?

      • Ventral intercostal branches

        • Anastomose with the ventral branches of the dorsal intercostal aa. and vv.

      • Perforating branches (supply cutaneous structures)

      • 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

          • Supplies skin over the rectus abdominis m.

          • 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)

      • Lateral cutaneous branches

      • 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.

      • Within the caudal mediastinum

    • Right phrenic n.

      • NOT within the caudal mediastinum b/c within the plica vena cava

  • Sympathetic trunk (paired)

  • 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)

      • Formed from fusion of caudal cervical ganglion and first 2-3 thoracic ganglia

      • 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

      • Vertebral n.

        • Branch that follows the vertebral a. through the cervical vertebra transverse foramina

      • Ansa subclavia

    • Middle cervical ganglion (paired)

      • Junction of the ansa subclavian and vagosympathetic trunk

      • 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)

      • Wraps around the ligamentum arteriosum (patent ductus arteriosis)

    • Right recurrent laryngeal n. (shorter)

      • Wraps around the right subclavian a.

    • Dorsal vagal trunk (unites more caudally)

      • Joining of dorsal branches of left and right vagus n.

    • Ventral vagal trunk

      • Joining of ventral branches of left and right vagus n.

Lymph nodes

  • Thoracic duct

  • Tracheobronchial lymph nodes (always present)

    • Left/Right

      • Located on the lateral surfaces of their respective primary bronchi

    • Middle (largest)

      • 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)


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