THE RABBIT. 331 



the retina, the blind spot or entrance of the optic nerve, 

 the ora serrata, the choroid processes, the lens, the 

 iris, the pupil, and the aqueous and vitreous humours : 

 all these have the same essential relations as in the lower 

 Vertebrata (see p. 84, 224228). 



344. The absence of a processus falciformis or 

 pecten (p. 122, 179, and p. 250, 314). 



345. The tapetum, a portion of the choroid which, 

 instead of having the usual black hue, exhibits iridescent 

 colours. 



XLII. Dissect away the parotid gland : clear away the 

 muscles, &c., from the entire external surface of 

 the bulla tympani ( 54) : lay open the external 

 auditory meatus by a longitudinal incision, until 

 the tubular portion of the tympanic bone is 

 reached : with bone-forceps cut away very care- 

 fully and gradually the outer wall of both tubular 

 and bulbous portions of the tympanic bone, 

 noting : 



346. The tympanic membrane, a transparent fibrous 

 partition, lying obliquely across the lower end of the 

 meatus: through it can be indistinctly seen a small bone 

 the handle of the malleus ( 73) attached to its inner 

 surface. 



347- The cartilage of the pinna, attached to the 

 tubular portion of the tympanic bone, and with it forming 

 the external auditory meatus, which thus consists of cartilagin- 

 ous and osseous portions. 



XLIII. Carefully cut away the tympanic membrane so 



as not to injure the manubrium mallei, and remove 



as much more of the tympanic bone as is necessary 



to display the following structures : 



348. The tympanum, or tympanic cavity, bounded 



