Anatomy of the Equine Thorax

Chapter 3: Anatomy of the Equine Thorax

Introduction

  • The equine thorax consists of 18 thoracic vertebrae that connect to 18 pairs of ribs.
  • Ribs connect:
    • Ventrally to the sternum at the costo-chondral junction.
    • Dorsally to the vertebrae.
  • The internal and external intercostal muscles also connect the ribs.
  • Key structures in the thorax:
    • Lungs
    • Heart
    • Mediastinum
  • Separation from the abdominal cavity is facilitated by the diaphragm.
Thoracic Cavities
  • The thorax is divided into three cavities:
    1. Thoracic cavity: Contains the lungs.
    2. Mediastinal cavity: Houses critical structures including the heart, trachea, esophagus, thymus (in young animals), aorta, and vena cava, along with tracheo-bronchial lymph nodes.
    3. Pleural cavity: This cavity is typically empty.

Auscultation of the Lungs

  • Auscultation is conducted in a triangular area defined by:
    • Ventral Border: A line connecting the olecranon process to the tuber coxae, delineating the basal border of the lungs.
    • Basal border extends from the 6th to the 11th ribs and the 16th intercostal spaces.
    • Cranial Border: A line connecting the muscles of the back with the olecranon process along the caudal border of the long head of the triceps muscle.
    • Dorsal Border: A line connecting the tuber coxae with the caudal border of the triceps muscle, aligning with the back muscles.

Lung Biopsy

  • Lung biopsy is performed on both sides of the horse.
  • Landmark for biopsy:
    • 7th or 8th intercostal space, located 3 inches dorsal to the line drawn from the olecranon process to the tuber coxae.

Liver Biopsy

  • Liver biopsy can also access the thoracic wall. This procedure is frequently utilized by equine practitioners.
  • Right side thoracic wall access:
    • Perform between the 12th and 14th intercostal spaces.
    • Area is bordered by two lines:
    1. The first line connects the olecranon process to the tuber coxae.
    2. The second line connects the point of the shoulder with the tuber coxae.

Auscultation of the Heart

  • The heart is primarily located on the left side, between the 2nd and 6th intercostal spaces.
  • Position and Orientation of the Heart:
    • Apex points towards the last segment of the sternum.
    • Base points towards the lungs.
  • Auscultation area spans:
    • 2nd to 6th intercostal spaces, with key points marking:
    • 3 (low) and 4 (high) on the left side.
    • 3 (low) and 4 (high) on the right side.

Skin Removal and Superficial Thoracic Vein

  • Upon skin removal from the thoracic wall, the superficial thoracic vein (often referred to as the "spur" vein) is visible.
  • Alternative venipuncture site for equine.

Muscles of the Thorax

  • After removing the skin and muscles:
    • Intercostal muscles, cutaneous trunci, and colli muscles become visible.
    • The thoracic cavity reveals that the lungs do not occupy the entire space, ending at the basal border (as noted earlier).
  • Clinical relevance of the basal border:
    • In live animals, it spans from the 6th to the 11th and the 16th intercostal spaces, but in embalmed horses, it shifts cranially.
Diaphragmatic Anatomy
  • The diaphragm separates the thoracic cavity from the abdominal cavity.
  • Potential injury to diaphragm:
    • Rupture can occur due to trauma, foaling, or intensive exercise leading to a diaphragmatic hernia, predominantly on the left side, which can cause colic.
  • Costo-diaphragmatic recess: Space between the diaphragmatic pleura and the costal pleura.
  • Line of pleural reflection: This line starts at the 9th intercostal space, moving caudally and dorsally to the 17th intercostal space, delineating the separation between the thoracic and abdominal cavities.
  • For thoracocentesis (aspiration of fluid from the thoracic cavity):
    • Needle placement should be two inches cranial to the line of pleural reflection in:
    • 6th or 7th intercostal space on the right side.
    • 7th and 8th intercostal space on the left side.

Required Structures for the Laboratory

  1. Auscultation of the lungs performed within a defined triangular area.
  2. Olecranon process as a landmark.
  3. Lung biopsy technique and location.
  4. Liver biopsy technique and location.
  5. Procedures for thoracocentesis.
  6. Long head of the triceps muscle as anatomical reference.
  7. Definition and significance of the basal border of the lungs.
  8. 6th, 11th ribs and 16th intercostal spaces as critical landmarks.
  9. 7th or 8th intercostal spaces as key locations for biopsy.
  10. Auscultation areas for heart between the 2nd to 6th intercostal spaces.
  11. Application of PAM and T (Pulmonary Artery Murmur and Tricuspid Area Murrmur) from small animals to equine.
  12. Features and implications of the superficial thoracic "spur" vein.
  13. Role of the external abdominal oblique muscle in thoracic anatomy.
  14. Presence and significance of the heave line in thoracic evaluation.
  15. Contributions of internal abdominal oblique muscle in thoracic stability.
  16. Function of the transversus abdominis in thoracic anatomy.
  17. Importance of the rectus abdominis muscle in thoracic assessment.
  18. Role of the diaphragm in separating cavities.
  19. Functions of the costo-diaphragmatic recess in thoracic health.
  20. Clinical significance of the line of pleural reflection in thoracic procedures.