Lesson 8 of 14
Objective: SWBAT identify the structures of a cow eye.
As the students enter the room, they take out their journals and respond to the prompt: Describe the path of light through the eye beginning with the cornea.
Some of the students have difficulty answering the question, so I remind them that they may refer back to their notes or textbook if they are unsure of some of the structures. I also circulate through the room reading the students' responses in order to have a better understanding of how well each of the students understands the information.
Once the students have had an opportunity to answer the prompt, we review it as a class. I take out our model of an eye and hold up the individual pieces as the students state the names of the structures. This review serves as a good reminder of the structures of the eye for the students as we prepare for the dissection.
This is the first dissection that my students have completed, so I spend extra time reviewing proper procedures with them as well as reviewing the tools they will be using during the dissection. I explain to the students that cutting through the eye can be difficult, and that they must be careful not to cut themselves or their group members. I have the students wear aprons, goggles, and gloves when they complete the dissection and I require students with long hair to tie their hair back. These specific rules have been in place throughout the year, so they are not new to the students. I explain to the students that they will primarily be using the scalpel, the probe, and the scissors for this dissection and that the dissection materials should be used only when the specimen is in the dissection pan. I also explain that since we are scientists, we will behave as scientists and avoid making comments such as "Yuck," "Gross," and for the most part, the students do a good job of avoiding these terms.
After reviewing the dissection information with the students, I ask them to repeat the rules back to me, sometimes multiple times, to ensure that they understand what they need to do while in the lab. If a student is not following proper procedure in the lab, that student is removed immediately, I have found that this greatly reduces off task behavior, since this is a lab the students look forward to all year.
This brief video provides some additional information on how I set up the eye dissection lab.
*Dissections are not for everyone, and I do not force my students to complete dissections. Please read my reflection for more information on how I use dissection alternatives with my students.
Once the students have their aprons and goggles on in the lab, I give each group a cow eye. I ask the students to look at the back of their eye dissection worksheet while I finish handing out the eyes. From there, I direct students to Eye Dissection (Page 1) and begin to work through the page with the students. I read the steps with the students and we complete the dissection together as a class. Staying together as a class is helpful because this is the first dissection for the students and it helps to ensure that the students are following the directions.
I use a sample eye to model the steps for the students and I carry this eye with me from group to group to demonstrate information as necessary. When I explain the step in which the students cut out the optic nerve, I try to use an analogy, to make it a little less disturbing and to provide students with a context they can understand. I compare the optic nerve to the stem of a pumpkin and I ask the students to think of how to turn a pumpkin into a jack-o-lantern. They are quick to respond that the area around the stem must be removed first. I explain that this is what they will be doing with the optic nerve. This is the most difficult part of the dissection, as the back of the eye is tough to cut through. I have the students make an initial puncture with the scalpel and then use their scissors to cut around the optic nerve. Many of the students still have trouble with making the initial incision, so I move from group to group, helping them accomplish this portion.
Once the students have removed the optic nerve, I ask them to try to look through the eye, as is listed on the dissection guide. Generally, the students are not able to see anything. Sometimes I veer away from the dissection guide and have the students take the lens out next. The lens is usually attached to the aqueous humor, so this part of the lab is very juicy. The easiest way for the students to remove the lens is to hold the eye with the hole where the optic nerve was downward while they use their thumbs to push in on the cornea. his part is rather juicy and if students are not careful, the lens may shoot out and bounce off of their dissection pan.
I have the students rinse off the lens once it is removed. I then ask them to examine it. The lens from our specimens are never clear enough to see through, but the students enjoy looking at them anyway. After the lens is removed, I have the students try to look through the eye again. Many times, they are able to see through the pupil.
I then move from group to group, asking the students to point out various structures of their eye. Sometimes I do this by pointing to a structure and asking students what it is and other times I ask the students to point to a structure that I name. Once the structure is located, I always ask the students to describe the function of the structure.
At the end of this portion of the lesson, I also provide students with a chance to explore the eye on their own. Many times students will use this time to try to cut the lens apart. This group exploration time provides the students with the opportunity to ask and answer their own questions about the eye.
Working through the dissection, we address NGSS MS-LS1-1 by exploring the different types of tissues that make up the eye and the types of cells necessary to form the tissues. We also address NGSS MS-LS1-8 and the CCC Systems and System Models when we examine the optic nerve and discuss how it connects the eye, as a system, to the brain, as a system.
Once the students have cleaned up their lab stations, I provide them with a little time to debrief and discuss the parts of the dissection they enjoyed and the parts they disliked.
I begin by asking them to talk to their group members about the experience. This ensures that each student has an opportunity to share information with others and have their voice heard. I then ask for volunteers to share their thoughts with the class. I also have the students share information about the parts of the dissection they disliked. During this part of the discussion, I do allow students to share more emotional responses ("It was cool." "It was gross."). Allowing a brief amount of time for this type of discussion makes it easier for students to focus on the next part of the discussion. For the whole class discussion I provide sentence stems to help the students focus back in on the topic. These stems include:
One thing that really stood out in the dissection was….
- When I was dissecting the …… I noticed that….
I also use more structure related questions, asking the students to describe the structures of the eye they were able to see and to explain what the structure looked like.