352 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



Cat: Remove the fleshy part of the external auditory meatus down to the 

 tympanic bulla. The meatus will be found to terminate at an oval opening with 

 a slightly elevated rim. Across the rim is stretched the delicate tympanic mem- 

 brane or eardrum. The handle of the malleus or hammer is visible through 

 the eardrum attached to its internal surface. Next, remove with the 

 bone clippers the ventral wall of the bulla. The interior is the tympanic 

 cavity of the middle ear. Note that it is divided by a bony plate into a 

 larger posteroventral chamber and a smaller anterodorsal chamber. The latter 

 is the one covered above by the tympanic membrane. Break open the plate of 

 bone, exposing this cavity, which is called the tympanum proper and which con- 

 tains the ear bones. Note the membrane which lines it and the eardrum forming 

 its anterodorsal wall. From the posterodorsal region of the cavity a calcareous 

 process projects toward fhe eardrum and carries the chorda tympani nerve, a 

 branch of the facial, to the eardrum. From the internal surface of the eardrum 

 the three little ear bones are plainly seen extending into a depression in the inter- 

 nal wall of the tympanum. These may be extracted and examined. Compare 

 with K, page 202, Figure 210. 



There now remains between the bulla and the cerebellar fossa the hard white 

 mass of the petrous bone already noted. This contains the internal ear. Owing 

 to the complexity of the internal ear and its small size, a dissection of it is imprac- 

 tical, but its main parts can be seen by breaking away the petrous bone in small 

 fragments. The tiny spirally coiled chamber in the bone is the cochlea; it 

 contains a spiral tube, the cotjilear duct, in which the organ of sound perception 

 (organ of Corti) is located. In the thicker harder part of the petrous bone are 

 semicircular channels, the semicircular canals, inclosing the semicircular ducts. 



The internal ear is thus seen to be inclosed in channels in the petrous bone, consisting 

 of the cochlea, the semicircular canals, and the vestibule or connecting chamber; together, 

 these constitute the bony labyrinth. The internal ear proper, or membranous labyrinth, is 

 contained in the bony labyrinth. Its parts are: the sacculus and utriculus inclosed in the 

 vestibule; the semicircular ducts arising from the utriculus and situated inside of the semi- 

 circular canals, and the cochlear duct arising from the sacculus and inclosed in the cochlea. 

 The cochlear duct is afrew structure characteristic of mammals, although it begins to appear 

 in birds. It is an outgrowth of the sacculus and is the real organ of hearing, the semicircular 

 ducts serving equilibratory functions only. 



5. The structure of the brain. — 



a) The membranes or meninges of the brain: With the two halves of the brain 

 previously removed before you, study the membranes of the brarn. The brain is 

 covered by a tough membrane, the dura mater. This consists of the dura mater 

 of lower forms fused to the internal lining (periosteum) of the skull. A consider- 

 able space, the subdural space, is present between the dura mater and the other 

 membranes of the brain. The dura mater dips down between the larger divisions 

 of the brain. The surface of the brain is covered by the delicate pia mater, in 



