THE PIGEON 187 



function as the three ear ossicles in the mammalian tympanic 

 cavity: they convey the sound waves from the tympanic mem- 

 brane to the inner ear. 



The tympanic cavity and Eustachian tube are homologous to the 

 first visceral cleft of fishes, which in the dogfish forms the spiracle. 



The Inner Ear. The principal organs of the inner ear which can 

 be seen in a dissection are the three semicircular canals. Carefully 

 shave away the bone in the auditory region and expose them. Two 

 of these canals are vertical in position, the anterior being the 

 larger and more dorsal ; the third canal has a horizontal position 

 and lies lateral to the other two. These canals spring from a small 

 sac called the utriculus, just beneath which is another small sac, 

 the sacculus ; projecting from the latter is the lagena, which corre- 

 sponds to the cochlea of mammals. 



Exercise 28. Draw a diagram showing the structures of the ear which 

 have been observed. 



The Brain and the Cranial Nerves. Entirely remove the remain- 

 ing skin and the muscles from the head and anterior end of the 

 neck ; remove the left eye. Beginning at its hinder side, cut 

 away with scalpel and scissors the roof of the skull, exposing the 

 dorsal surface of the brain. The skull is easily cut with a scalpel 

 because of its spongy nature. It is very thin, and the brain en- 

 tirely fills the cavity. Care must be taken, consequently, not to 

 cut too deeply. Cut away also the upper half of the orbit and 

 the side of the skull behind it. 



In its dorsal aspect the brain will be seen to be made up of five 

 divisions, of which two, the cerebrum and the cerebellum, project 

 prominently and form almost its whole dorsal surface. The an- 

 terior division is the cerebrum, which is composed of the two large 

 hemispheres; at their anterior ends are the two small olfactory 

 lobes. Immediately behind the hemispheres and between them 

 and the cerebellum is the delicate pineal body, or epiphysis, which 

 is a projection of the dorsal wall of the second division of the 

 brain, the diencephalon. 



The third division, or midbrain, is composed dorsally of the 

 optic lobes, which appear as a pair of spherical bodies at the sides 



