Inner Organs: Thoracic Cavity

In my previous post, I accidentally omitted the procedure for removing the rib cage. In order to reach the tracheobronchial lymph node, and the other organs of the thoracic cavity, this step is necessary. The abdominal organs are first exposed by making an incision on the abdominal wall mid-ventrally at the site of the last rib. Cutting cranially along the thoracic arch, reflect the abdominal musculature as you go along. The thoracic cavity is then opened by trimming away muscle attached to the rib and deflating the diaphragm by puncturing it. Note if there is deflation, as an absence of deflation can suggest severe pneumonia. It is important to note that the diaphragm is a large structure that separates the thoracic cavity from the abdominal cavity, and once punctured, will deflate healthy lungs due to positive ambient air pressure invading a previously air-proof cavity. The rib cage is then removed by cutting through each thoracic rib mid-articulation (also known as a sweet spot). This is a flex point made of cartilage whose purpose is to allow movement during breathing. Cutting cranally, the ribs should also be reflected, revealing the thoracic cavity.

Pinnipeds have comparatively larger thoracic cavities than their terrestrial distant relatives. This was especially apparent during my last necropsy on a gray seal (Halichoerus grypus), as that specimen's neck was almost as long as the rest of its body. This is congruent with the seal's opportunistic diet, as it must be able to seize any size of fish possible. The esophagus found in pinnipeds has the attributes of extreme flexibility and elongation, a necessity to process large prey.

Organs in the thoracic cavity can be removed and examined separately, or be removed together. The latter situation involves removing the tongue, larynx, trachea, esophagus, bronchi, lungs and heart and is termed the pluck. This procedure enters through the head, as skin is peeled off the chin and neck and then the sides of the mouth are cut open, the incisions extended caudally to the thorax. While pulling the tongue, the connective tissue and muscle holding the pluck is snipped until everything falls loose. I witnessed this on my first necropsy of a harp seal (Phoca groenlandica) and it was performed by a veterinarian. Thus it is my guess that until a vast amount of practical experience is accrued, I will not attempt this particular method. Maybe someone will prove me wrong..

As noted before, healthy lungs may collapse when the diagraphm is deflated. These organs are large and conspicous in the thoracic cavity, and will normally have a bright pink color and sponge-like texture. The lungs can be detached at the bifurcation of the trachea and then examined. Pressing down on the lungs, gauge if they bounce back, as normal lung tissue will act sponge-like. Additionally, healthy lungs should float if placed in water or a formalin solution. Translucent connective tissue connects lobules in pinniped lungs, and this tissue can become filled with gas in a pathological condition known as emphysematous. Using scissors, cut along the bronchioles of each lung and note if there are parasites, fluid, etc. Next, by bread-slicing you can further eamine the tissue. Make parallel slices perpendicular to the long axis of the body, and is best accomplished by a swift and single cut. This will allow you to view the bulk of the organ (parenchyma) and note any deviations from normal color/texture without any tears or serrations. The trachea should also be examined by cutting longitudinally along the entire length and then examining any mucus or other fluids.

My next post will deal with the heart. I am consulting further texts beyond the necropsy guides to further my knowledge of the heart because I honestly don't have a full grasp of the mammalian heart. Until then, ciao.