RPQ 5 - carrying out dissections

• dissection tools:

  • scalpel
  • dissecting scissors
  • tweezers/ forceps
  • dissecting pin
• scalpels have very sharp detachable blades and can be used for making very fine cuts
• dissecting scissors are also used for precise cutting - safer to use than scalpels bc the blades are less likely to snap under pressure and it can be easier to avoid damaging the tissue underneath when using scissors
• dissecting pins can be used with a wax-filled dissection tray to pin a specimen in place during the dissection
• you could carry out the dissection on a wooden cutting board instead of a dissection tray
• tweezers are useful for holding and manipulating the smaller parts of the specimen
• dissecting tools should all be clean, sharp and free from rust - blunt tools do not cut well and can be dangerous
• dissecting animals including fish and insects can give a better understanding of their anatomy - some ethical issues involved
• some people argue it is morally wrong to kill animals just for dissections - unnecessary killing. however many dissections that take place in schools involve animals that have already been killed for their meat - this is not a problem - some people disagree with killing animals altogether
• there are concerns that the animals used for dissections are not always raised in a humane way - they may be subject to overcrowding, extremes of temperature or lack food - may not be killed humanely either
• if animals e.g. insects are raised in a school for dissection it is important to make sure they are looked after properly and killed humanely to minimise any suffering or distress
• lung dissection - messy - wear lab coat - if cutting cartilage - wear goggles - wear goggles anyway
• 1. lay lungs on the cutting board - should be able to see trachea and 2 bronchi going into lungs
  2. to see the lungs inflate, place them in a clear plastic bag, attach a piece of rubber tubing to the trachea and pump air into the lungs using a foot or bicycle pump - the lungs will deflate by themselves bc of the elastin in the walls of the alveoli
• never blow down the tube to inflate the lungs - could end up sucking up stale air from inside the lungs into your mouth

  putting the lungs in a plastic bag stops bacteria inside the lungs from being released into the room
• 4. once seen the lungs inflate, can examine different tissue types in the lungs
• 5. trachea is supported by C-shaped rings of cartilage

  6. cartilage is tough, so if want to open the trachea best to cut it lengthways down the gap in the C-shaped rings

  use dissecting scissors or a scalpel to make the cut

  if using a scalpel cut downwards (not towards you) and don't apply too much pressure to the blade
• • 7. continue cutting down one of the bronchi - should be able to see the bronchioles branching off
  • 8. cut off a piece of the lung - the tissue will feel spongy bc of the air trapped in all the alveoli
  • 9. lungs from a butcher are safer for a human to handle, but could still contain bacteria that cause food poisoning - wash hands after dissection and disinfect work surfaces
• dissection of a bony fish:

  1. wear lab coat or apron
  2. place fish in a dissection tray or on cutting board
  3. gills are located on either side of the fish's head - protected on each side by a bony flap called an operculum and supported by gill arches
  4. to remove the gills push back the operculum and use scissors to carefully remove the gills. cut each gill arch through the bone at the top and bottom
  5. if look closely should be able to see gill filaments - with gills above - not possible to see lamellae without microscope
• dissection of a large insect:

  1. big insects like grasshoppers or cockroaches are usually best for dissecting - easier to handle - humanely killed recently
  2. fix insect to dissection tray - put dissecting pins through legs to hold in place
  3. to examine tracheae need to carefully cut and remove a piece of exoskeleton (hard outer shell) from along length of insect's abdomen
• • 4. use a syringe to fill the abdomen with saline solution - should be able to see a network of very thin, silvery-grey tubes - these are the tracheae - look silver bc filled with air
  • 5. can examine tracheae under optical microscope using a temporary mount slide - again tracheae will appear silver or grey - should be able to see rings of chitin in walls of tracheae - there for support (like rings of cartilage in human tracheae)
• heart dissection:

  1. wear lab coat and lab gloves
  2. place heart on dissecting tray
  3. look at outside of heart to identify 4 main vessels attached to it - feel inside vessels to help - arteries are thick and rubbery whereas veins are much thinner
  4. identify right and left atria, right and left ventricles and coronary arteries - might be asked to draw sketch of outside of heart and label
• • 5. using clean scalpel, carefully cut along outside edges to look inside each ventricle - could measure and record thickness of ventricle walls and note any differences between them
  • 6. next cut open atria and look inside - note whether atria walls thicker or thinner than ventricle walls
• • 7. find the atrioventricular valves, followed by the semi-lunar valves - look at structure of valves and see if can see how they only open one way - could draw a sketch to show valves and inside of ventricles and atria
  • 8. make sure wash hands and disinfect all work surfaces once completed dissection
• Plant mass transport dissection: - can look at xylem or phloem in plant tissue (e.g. part of a plant stem) and then draw them - first need to dissect plant and prepare a section of the tissue
• Observing xylem and phloem in a stem

  1. Use a scalpel or razor blade to cut a cross-section of the stem

  2. Cut the sections as thinly as possible

  3. Use tweezers to gently place the cut sections in water until you come to use them

  4. Add a drop of water to a microscope slide, add the plant section and carefully add 1/2 drops of a stain e.g. toluidine blue O and leave for 1 min

  5. Carefully apply a coverslip so have created a temporary mount
• Viewing the specimen under a microscope

  • If used TBO should be able to see the xylem cells stained blue-green
  • Phloem and rest of the tissue should appear pinkish purple
• the arrangement of the xylem and phloem in a cross-section of a stem (of a non-woody plant) - phloem on outside of xylem