74 LABORATORY DIRECTIONS 



the Eustachian tube, connecting with the mouth cavity, are 

 homologous to the spiracle of the shark. 



Mammalian Ear 



Study charts and models of the human ear and be able to 

 identify the various parts. Notice particularly the three bones 

 of the middle ear (malleus, incus, and stapes) and the cochlea of 

 the inner ear. From what are the three bones of the middle 

 ear derived ? Be sure that you understand how the ear 

 functions. 



BRAIN AND CRANIAL NERVES 

 Brain of the Shark — Dorsal Surface 



Slice away the cartilage of the roof of the cranium so as to 

 expose the brain. Nerves will become visible as dissection 

 proceeds. Try not to cut them. They will be identified 

 later. 



The brain and spinal cord are covered by a thin membrane, 

 the -primitive meninx, which is comparable to the meninges 

 (three membranes) of mammals. 



The large anterior part of the brain is the telencephalon. It 

 is composed of indistinctly paired cerebral hemispheres, and in 

 front of these, large swellings, the olfactory lobes. Extending 

 forward from the olfactory lobes are short stalks, the olfactory 

 tracts, which end in olfactory bulbs. Short olfactory nerves 

 extend from the olfactory bulbs to the nasal capsules. The 

 olfactory lobes, tracts, and bulbs constitute the rhinencephalon. 



Posterior to the cerebral hemispheres there is a narrow 

 depressed region, the diencephalon. A thin layer of blood 

 vessels, known as the choroid plexus, covers the third ventricle. 

 The epiphysis, a slender thread-like structure, arises from this 

 part of the brain. It was probably destroyed in removing 

 the roof of the cranium. 



The mesencephalon or midbrain lies behind the diencephalon. 

 It consists mainly of two large optic lobes. 



The cerebellum or mete7icephalo7i lies posterior to the optic 

 lobes and somewhat overlaps them. Shallow cross lines give 

 the cerebellum the appearance of being divided into four 

 parts. 



