DISSECTION OF ANIMAL SPECIMENS

Dissection of the following is required in A-level.

1. Rat
2. Toad frog (Amphibian)
3. Cockroach (Insects)

Aims for dissection

1. To strengthen one understands of general anatomy of these organisms, how their organs function and interrelationship between position (location), shape etc with others in system where they operate with others.
2. Train student to be able to follow instructions, handle apparatus or instruments, draw and label.

Mastering skills of dissection

Dissect the actual specimens and display the systems they are in. draw and label as many things as possible.

Precautions to good dissections

1. Always use clean sharp cutting instruments
2. Stick to the instructions and learn to memorize them incase they are nor provided.

• Avoid bleeding incase of rats and frogs by being careful and use right instruments.
• 1. Incase of bleeding, mop up blood immediately with cotton wool. Do not panic but be speedy and accurate.
  2. Do not cut anything until you are sure that what it is.

  Always dissect along and not across. After dissection, draw

DRAWING

1. Use a sharp Hb pencil always, this gives drawing, of uniform thickness.
2. The drawing must not be artistic but should be biological, i.e. do not shade or colour your diagrams. Accuracy of drawing.

• Pay attention to position, number, sizes, proportions, and shapes of each component part of the specimen you are drawing. Marks will be lost without this; e.g. where joint exist, pay attention to their shapes as of the insect legs, antennae etc.

1. Parts attached to major specimen e.g. legs, hairs; spines should be drawn touching without any gap between them and the specimen. The gaps left appear to be hanging.
2. Make sure you are drawing the right view of system i.e. dorsal, ventral, anterior, posterior, lateral, left or right of the specimen etc.
3. 1. Draw directly without first making a sketch.

Outline

1. Always produce an outline of the whole animal or part of it in the region dissected, this shows the location of the area dissected.

Make sure you show clearly each and every relevant organs and structures to the system displayed. Blood vessels or organs may be superposed on each other in this case avoid crossing lines

1. A smooth outline no gaps/blockages, wary outlines earn no marks.

Use of double lines

Double lines must be used for tubular structures, like blood, intestine etc.

It the drawing is not completed, close up gaps or ears of ligature, use X or – e.g

Use of single lines

These are used for drawing nerves.

Size of drawing

Use three quarters of the space provided.

The title and labeling

Every drawing must have a title obtained form the instructions. It must be specific, relevant and state the specimen e.g. if the specimen is called specimen X, stick to using X. Use label lines i.e. ____ but not arras i.e.

Technical terms must be correctly spelt or interpreted e.g. nucleus vs mitochondrion vs mitochondria, ceacum vs ceaca.

Interpretation. Vessels that suck blood to (arteries), vessels that drain blood from.

• Do not eras levels.

• Label in singular if the label is touching one part and in plural if touching more than one.
• Do not label in the drawing, if this is done all the others are erased.
• Do not label what is not part of the question e.g. if required to draw the arterial system of the left limb; if you label that of the right, all will b crossed and you earn a mark for the drawing only.

Going beyond what is wanted earns you zero. If requested to label any 5, do not exceed

Magnification

The magnification must be stated which must have a bearing on the space given.

Use three-quarters of the space for drawing which must be X 1 or X 2 for a rat, and X1, X2, X3 for a toad.

Neatness

– Takes into consideration of everything in the drawing e.g. lines, labels, etc.

Time limits.  Maximum time spent on drawing = 15 minutes. The rest 45 minutes are for.  

Adventurism should be encouraged.

In summary, marks are awarded in the following for any drawing represented at the dissection;

1. Title
2. Drawing = Correct view, proportion, No, position, correct size, sharp pencil, outline, continuous lines.

• Labeling = Any labeled organ not part of the required system will make you lose all the marks for other labels i.e. you earn zero.

1. Neatness
2. Magnification
3. Outline

Transport. Unless you are comparing 2 or more statements, do not write negative statements if given only specimen eg lacks tail.

Observation. The following are important.

• Pasture and relative numbers} Topographical relative position.
• Size (volume, shape, length, breath, surface area).
• Give exact measurements.

1. Shape
2. Colour

• Appendgages (numbers, pattern, etc)

1. Nature of surfaces and borders (ie smoothness/roughness/stiffness stickiness

• Dry/moist/wet
• Burnt/sharp border

• Warty (with secreral swellings etc.
• Always differentiate between structural and functional differences.
• A lot of practice at drawing and labeling.