324 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



of skin. Remove this piece of skin and find beneath it a smaller cartilaginous 

 circular plate, the tympanic membrane or eardrum. Make a cut around the 

 margin of this and carefully raise it. Attached to its internal surface, posterior 

 'to the center, is a rod-shaped bone, the columella, whose inner end is fastened to 

 the wall of a large cavity. This cavity is the tympanic cavity or cavity of the 

 middle ear. It is an evagination from the first visceral pouch. Ventral to the 

 inner end of the columella is a slit bounded by raised lips. This slit is the open- 

 ing of the auditory tube connecting the pharyngeal cavity with' the cavity of 

 the middle ear and representing the stalk of the evagination by which the latter 

 was formed. Considerably internal to the point of attachment of the inner end 

 of the columella lies the internal ear. It will be more definitely located later. 

 It is similar in structure to the internal ear of elasmobranchs. 



3. Dorsal aspect of the brain. — Remove the roof of the skull and expose the 

 brain. The brain is covered by a tough membrane, the dura mater. On cutting 

 carefully through this a more delicate membrane, the pia mater, will be found 

 adhering to the brain. It is more or less pigmented and contains numerous blood 

 vessels. Strands cross between the two membranes. The two membranes are 

 derived by the splitting of the original single membrane of the brain, the primitive 

 meninx of fishes. The space between the dura mater and pia mater is the sub- 

 dural space; between the former and the skull, the peridural space. Remove the 

 dura mater from the dorsal surface of the brain. 



The brain possesses the same divisions as in the preceding forms, but the 

 relative proportions of the parts are somewhat altered. The anterior end con- 

 sists of the two olfactory lobes, which include the olfactory bulbs and tracts seen 

 in the elasmobranch ; the whole olfactory apparatus is evidently much reduced 

 in the turtle as compared with the fish. Posterior to the olfactory lobes and 

 separated from them by a slight groove are the two cerebral hemispheres, relatively 

 much larger than in the elasmobranchs and Nee turns. Between the posterior 

 ends of the two cerebral hemispheres is the diencephalon, its roof consisting of 

 a chorioid plexus, which projects dorsally as a sac that adheres to the dura mater 

 and is generally torn off in removing the latter. This sac consists of the 

 paraphysis and the epiphysis. When it is removed the diencephalon is seen 

 as a depressed area posterior and ventral to the cerebral hemispheres. Posterior 

 to the diencephalon are the two rounded optic lobes, belonging to the midbrain. 

 Behind them is the cerebellum, smaller than in elasmobranchs but larger than 

 in Amphibia. Posterior to the cerebellum and partly overhung by it is the me- 

 dulla oblongata. Its thin dorsal roof contains as usual a chorioid plexus. On 

 removing the roof of the medulla, the cavity of the fourth ventricle is revealed 

 and the cerebellum is then seen to project like a roof over the anterior part 

 of the medulla. The dorsal rims of the medulla consist as usual of the somatic 

 sensory columns, and on raising the cerebellum the anterior ends of these col- 

 umns which are auditory in nature are seen to be connected with the cerebel- 



