330 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



and forming the iris in front; the soft retina. Note the peculiar ridged structure, 

 the pecten, projecting from the chorioid coat through the retina in the medial 

 wall of the eyeball and extending to the lens. Its function is unknown. Loosen 

 the lens and observe that it is encircled by a structure formed chiefly from the 

 chorioid coat. This structure, which holds the lens in place, is called the ciliary 

 body. It is marked by radiating ridges, the ciliary processes, and contains 

 muscles, the ciliary muscles, which change the shape and position of the lens. 

 Note the shape of the lens, nearly flat externally, more convex internally. The 

 chambers of the eye and the two humors are as in the dogfish. Peel off the iris 

 from the cornea and note the stiff, bony ring encircling the cornea; it is composed 

 of a number of sclerotic bones. 



Draw, showing structure of the eye. 



c] The ear: The ear of birds consists of three parts the external ear, the 

 middle ear, and the internal ear. The external ear comprises the passage, the 

 external auditory meatus, situated below and behind the eye. Cut into this on 

 the same side of the head on which the eye was dissected and find at its internal 

 end a circular transparent membrane, the tympanic membrane. Through the 

 membrane the columella can be seen extending from its internal surface inwardly. 

 Remove the tympanic membrane, noting the columella adhering to its internal 

 surface. The cavity of the middle ear is now exposed; medially and ventrally 

 it is connected to the pharnygeal cavity by the auditory tube; posterior and 

 slightly dorsal to it is situated the internal ear. The inner end of the columella 

 adjoins a tiny bone, the stapes, which fits into an opening, the fenestra ovalis or 

 vestibuli, which leads into the internal ear. Look for these at the inner end of 

 the columella. Next, carefully break away in small pieces the spongy bone 

 behind the middle ear. Three bony semicircular canals are revealed. Each of 

 them contains a membranous semicircular duct, as will be seen by breaking open 

 one of them. The three ducts are situated in the same planes and have the 

 same names as in elasmobranchs. The remaining structures of the internal ear, 

 consisting of two small chambers the utriculus and the sacculus are too difficult 

 to dissect. 



3. Dorsal aspect of the brain. Expose the brain, removing the roof of the 

 skull and the side of the skull where the sense organs were dissected, including 

 the medial wall of the orbit. Note: the dura mater inclosing the brain; on 

 removing this, the very delicate pia mater next to the brain substance. The 

 origin of these membranes is the same as given for the turtle, and the spaces 

 bounding them have the same names. Unlike the condition in the preceding 

 forms, the brain completely fills the cranial cavity. 



The brain is short and broad and strongly curved, in correlation with the 

 biped gait. The curvature results from flexures of the brain in three regions. 

 The chief or primary flexure occurs in the region of the midbrain, with the result 

 that the posterior part of the brain is bent nearly at right angles to the anterior 



