Lab: Rodent Dissection

Specimen Identification and Taxonomic Distinctions

• Rodent Identification:

  • Chipmunks: Tiny animals with highlighting around their eyes.
  • Squirrels:
    • Tree Squirrels: The primary example provided is the fox squirrel, described as "regular scampus squirrels."
    • Ground Squirrels: The collection includes examples of male, female, and juvenile specimens, collected at different times to ensure an entire family was not destroyed.
  • Rats vs. Mice: The primary distinguishing factor between them is physical size; the rat is the larger specimen, and the mouse is the smaller one.

• Taxonomic Genus Names:

  • Rats: The genus for rats is Rattus.
  • Mice: The genus for mice was noted by the speaker as Mus (the transcript phonetically records this as "moose").

• Groundhogs/Marmots:

  • A marmot is identified as a type of groundhog.
  • Vocalization: Contrary to popular internet memes, marmots do not actually scream with human-like vocalizations. Their actual scream sounds more like a short, sharp "a."

Laboratory Procedures and Safety

• Hygiene and Chemical Exposure:

  • Specimens are often treated with mothballs or other preservatives. Students are strictly instructed to wash their hands immediately if they touch the specimens.
  • Specimens generally have a strong odor due to chemical preservation.

• Surgical Techniques:

  • Tool Selection: Scissors are recommended over scalpels for initial incisions. Scalpels have a tendency to cut too deeply, which can damage internal structures.
  • Dissection Trays: These should be obtained before beginning the procedure.
  • External Cleanup: If there is excess fluid in the back of the specimen, it should be dried with a paper towel to visualize structures like the ureters.

• Taxidermy:

  • The specimens in house (like the fox squirrel) were prepared using taxidermy by the personnel, with the fox squirrel noted as the first one ever completed, showing high detail.

External and Internal Anatomy: The Neck and Throat

• The Throat Hierarchy:

  • The anatomical order from top to bottom is: Pharynx $\rightarrow$ Larynx $\rightarrow$ Trachea.

• Pharynx:

  • This is the throat area located above the larynx.
  • It is described as being relatively short, ending where the throat meets the chin if the cut is extended upward.

• Larynx (Voice Box):

  • Located below the pharynx, the larynx is a wider structure.
  • It contains two visible white dots.
  • The larynx is notably wider in male specimens.
  • Thyroid Gland: Two red patches located right next to the white dots on the sides of the larynx. It is described as being thin, like "a little thin piece of beef jerky," and tapered directly onto the larynx.

• Trachea (Breathing Tube):

  • A long tube located below the larynx.
  • Structure: It contains cartilage rings that prevent the tube from collapsing when the neck bends.
  • The Cartilage C-Shape: The cartilage does not form a complete circle; it is a C-shape. The front side is protected by cartilage, but the back is soft/open to accommodate the esophagus.

• Esophagus:

  • Located directly behind (deep to) the trachea.
  • Functional Implication: Because the esophagus is directly behind the trachea, swallowing a very large object causes the esophagus to expand and press against the trachea, which can lead to choking.

Thoracic Cavity: Thymus, Heart, and Lungs

• Thymus:

  • Described metaphorically as a "wizard hat" sitting on top of the heart.
  • It is a separate organ that can be lifted to reveal the top of the heart.

• Heart (General):

  • Located in the chest cavity, centered behind the thymus.
  • The model used for the heart is significantly larger than the actual rat heart to facilitate study.

• Lungs:

  • Located on either side of the heart.
  • Lungs consist of multiple lobes; specifically, there are multiple lobes for the right lung and a separate left lung.

• Diaphragm:

  • A muscle that separates the chest (thoracic) cavity from the abdominal cavity.

Abdominal Cavity: Liver, Stomach, and Digestion

• Liver:

  • The largest organ in the abdominal cavity, consisting of multiple lobes.
  • It is substantial because rats consume a variety of materials and need robust detoxification systems.

• Stomach:

  • Located beneath the liver. The liver must be lifted to fully visualize the stomach.

• Pancreas:

  • Unlike the solid lobes of the liver, the pancreas appears as a mesh or sheet of yellowish, "globby" material.
  • It is located in the mesentery between the stomach and the small intestines.

• Spleen:

  • Located on the left-hand side of the stomach.
  • It is described as looking like a "sideways tongue" and is part of the lymphatic system.

Intestinal Systems and Comparative Anatomy

• Mesentery:

  • A sheet of connective tissue that holds the intestines in place.
  • Contains blood vessels, specifically the Anterior Mesenteric Arteries. These arteries often appear clear because they have been drained of blood.

• Small Intestines:

  • The portion of the digestive tract located between the stomach and the cecum.

• Cecum:

  • A large, pouch-like structure marking the end of the small intestine and the beginning of the large intestine.
  • In rats, the cecum is extremely large due to their diet.
  • Evolutionary Homology: The rat's cecum is homologous to the human appendix. Humans have a small appendix because they no longer consume the rough diet that requires a massive cecum.

• Large Intestine and Rectum:

  • The large intestine begins at the cecum and ends at the anus.
  • Rectum: The very last part of the large intestine. In rats, it may contain fecal "pellets" ready for excretion.

Male Reproductive System

• External Anatomy:

  • Scrotum: The external skin sac located at the posterior end of the rat.

• Internal Anatomy:

  • Testes: Located inside the scrotum. When dissected, they are cut lengthwise (like a "hot dog" or "papaya").
  • Seminiferous Tubules: Tiny tubes found inside the testes where sperm is produced.
  • Epididymis: A fatty cap found on both the top and bottom of the testes where sperm travels after the seminiferous tubules.
  • Vas Deferens: A cord/duct that carries sperm from the epididymis. It travels up and loops around the bladder.
  • Seminal Vesicles: Large, textured structures located behind the bladder, often described as "angel wings."
  • Prostate Gland: A gland that surrounds the base of the bladder.
  • Penis: Often located in the mid-line; during dissection, it may accidentally be removed when opening the specimen.

Female Reproductive System (Comparative)

• Uteri: Rats have a "bicornuate" uterus, meaning they have two uteri (horns) rather than a single uterine body like humans.
• Vagina: The point where the two uteri connect at the posterior end.
• Ovaries: Located at the anterior end of the uterine horns, appearing as small "tags" or structures.
• Oviducts: Small tubes that connect the ovaries to the uterus.

Urinary System: Kidney Structure and Function

• Kidneys:

  • Two bean-shaped organs located against the back wall of the abdominal cavity.
  • Renal Cortex: The outer shell or layer of the kidney.
  • Renal Medulla: The middle portion of the kidney, composed of Renal Pyramids.
  • Renal Pelvis: A funnel-like structure that collects urine from the kidney before it enters the ureter.

• Ureters:

  • Extremely thin tubes that drain urine from the renal pelvis of each kidney down to the bladder.

• Bladder:

  • A small, bean-shaped sac that stores urine.

Major Circulatory Structures and the Heart Model

• Atria and Oracles:

  • Atria: The two upper chambers of the heart (Right and Left Atrium). They receive returning blood.
  • Oracles: Folded, flap-like structures on the atria. They allow the atria to expand (plump up) if they overfill with blood, preventing the chamber from bursting.

• Ventricles:

  • The two lower chambers of the heart (Right and Left Ventricle).

• Deoxygenated Pathway (Right Side):

  • Blood from the upper body enters via the Superior Vena Cava.
  • Blood from the lower body enters via the Inferior Vena Cava.
  • Both empty into the Right Atrium.
  • Blood moves to the Right Ventricle, which pumps it through the Pulmonary Trunk to the Pulmonary Arteries (leads to the lungs).

• Oxygenated Pathway (Left Side):

  • Blood returns from the lungs via four Pulmonary Veins.
  • Pulmonary veins drain into the Left Atrium.
  • Blood moves to the Left Ventricle, which pumps it through the Aortic Semilunar Valve into the Aorta.

• The Aorta and Branching Arteries:

  • The Aorta is a continuous tube that arches and then descends down the back.
  • Three Main Branches from the Aortic Arch:
    1. Brachiocephalic Artery (Right side): A single tube that splits into the Right Common Carotid (going to the neck/head) and the Right Subclavian (going under the clavicle to the right arm).
    2. Left Common Carotid Artery (Middle): Goes straight up the left side of the neck to the head.
    3. Left Subclavian Artery (Left side): Goes under the left clavicle to the left arm.

• Additional Vessels:

  • Coronary Arteries: Smaller arteries located on the surface of the heart itself that supply the heart muscle with blood.
  • Jugular Veins: Two veins on each side of the neck that bring deoxygenated blood from the head back to the Superior Vena Cava.
  • Renal Artery: A red artery branching off the aorta that supplies the kidneys with blood.

Questions & Discussion

• Disease Concerns: A student mentioned "Hunter virus," which the instructor clarified as Hantavirus, a pathogen often associated with rodents.
• Exam Logistics: Students were told they could pick up the heart models during the exam and hold them to their own chests to help identify "left" and "right" orientations.
• Medical Anecdotes: The instructor and students discussed news reports, including a case of an automated medical facility in New York where a patient died because there was no physical personnel to intervene during a code, highlighting the risks of removing human oversight in medicine.