Comparative Vertebrate Anatomy Lab 6: Brain & Cranial Nerves of the Shark Lab Objectives 1. To learn the component parts of the shark’s brain and its cranial nerves. 2. To study the extrinsic muscles of the eye and how they work to move the eye. 3. To learn the component parts of the sheep’s brain 4. To gain an appreciation for the organization of the brain. Material to Learn Shark brain and cranial nerves, Shark eye and Sheep brain • Figures 3.36, 3.37, 3.40 are excellent for giving a variety of views • Figure 3.41 contains almost everything that you need to show in your dissection • Associated text: pp. 64-­‐75 • Figures 7.73-­‐7.77 Shark Term list Anterior semicircular Posterior semicircular Infraorbital n. duct Iris duct Aqueous humor Lateral rectus m. Pupil Auricle Lens Retina Cerebellum Mandibular n. Sacculus Cerebrum Medial rectus m. Sclera Spinal cord Cornea Medulla oblongata Deep ophthalmic n. Oculomotor n. Statoacoustic n. Dorsal oblique m. Olfactory bulb Superficial ophthalmic Dorsal rectus m. Olfactory lobe n. Facial n. Trigeminal n. Olfactory sac Geniculate ganglion Olfactory tract/nerve Trochlear n. Vagus n. Glossopharyngeal n. Optic lobe Horizontal Ventral oblique m. Optic chiasm semicircular duct Optic n. Ventral rectus m. Hyomandibular n. Vitreous humor Optic pedicle Hypophysis Petrosal ganglion Background & Instructions During this week’s lab you will dissect the shark’s brain and associated cranial nerves and the sheep’s brain. Be very careful as you do this dissection so as not to cut into structures that you want to preserve. Your goal is to be able to show all of the structures that are included in the term list and shown in Figure 3.41 of your manual. However, do not dissect as far posteriorly as is shown in figure 3.41; go only to the point where the medulla oblongata becomes the spinal cord, a little anterior to the point where the probe is drawn in the figure. I. Shark dissection instructions 1. Skin the dorsal surface of the head completely using the scalpel. Skin from the tip of the rostrum to a point approximately 2 cm posterior to the gill slits. In places, you will find that skin closely adheres to the underlying chondrocranium. Feel free to use the scalpel to remove the skin from these places without much fuss – if you shave off a bit of cartilage, it’s not a big deal, but stay superficial so that you don’t cut into the structures you are trying to preserve. 2. Start cutting away the cartilage of the chondrocranium to reveal the brain and cranial nerves. Do this cautiously – the cartilage is much more dense than the brain and nerves and so slipping through it can lead to stabbing the brain. Use the scalpel to shave off thin bits of cartilage. As you do this, watch for slight changes in the color of the cartilage. When the intact cartilage is very thin, it is translucent, and a change in color often indicates that you’re very close to the cavity holding the brain. 3. A good place to start shaving cartilage is mid-­‐dorsally, where you can see/feel a bump in the chondrocranium. This overlies the cerebellum, which is closest to the surface. Once you get through the cartilage there, expand to remove more cartilage and expose more of the brain. Work from the middle outward. As you move peripherally, you’ll have to cut away pieces of muscle and that is fine. However, be careful of the hyomandibular n. as you clean off any of the adductor mandibulae m. Also watch out for the semicircular ducts, which are cartilaginous, so the same color as the chondrocranium (again, you will see shadowing as you get close to the semicircular canals). Identify the semicircular 3. Make some finishing touches. Smooth out jagged shards of cartilage. Remove one of the auditory apparati – but keep this for last because if you cut into one, you can always do the other side if it is still intact. Also remove one of the eyeballs and identify the structures in the term list.(See figures 3.38 & 3.39). II. The cranial nerves Identify the nerves in your shark specimen. The cranial nerves are segmental nerves, serially homologous to the spinal nerves, but they exit the brain instead of the spinal cord. The cranial nerves innervate many sensory structures in the head, eye muscles, and the pharyngeal arches (one cranial nerve per arch). As such, these nerves can have sensory neurons in them, motor neurons, or both, and so the cranial nerves can be categorized into sensory, motor, or mixed. Like spinal nerves, some cranial nerves have ganglia. A ganglion is a slight swelling along the length of the nerve where the cell bodies of the neurons reside. As you identify the nerves remind yourself what each one enervates. What is the action of the following extrinsic eye muscles? (Hint, which way does each one make the eyeball move?) Dorsal oblique m. – Medial rectus m. – Ventral oblique m. – Ventral rectus m. – Lateral rectus m. – Dorsal rectus m. -­ III. Sheep Brain Examine the whole and sagittal sheep brains, identify the following and state which of the five regions of the brain each component belongs to. Arbor vitae Mamillary body Cerebrum Medulla oblongata Cerebellum Olfactory bulbs Corpus callosum Optic chiasma Fourth ventricle Pineal Gland Fornix Pituitary (Hypophysis) Hypothalamus Pons Infundibulum Superior Colliculi Longitudinal Fissure Take a look at the comparative brain display and describe the differences that you see in the relative size of the components of the brain in the representative taxa.