344 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



communicate with the surrounding cellular part of the tympanum. The fora- 

 men rotundum, which is the beginning of one of these turns, appears to be 

 only one end of a transverse groove, which is afterwards closed in the middle, 

 forming a canal with the 2 ends open ; so that this foramen appears to have 2 

 beginnings; but the other opening is probably only a passage for blood-vessels 

 going to the cochlea. From this foramen begins the inner turn of the cochlea, 

 which is the largest, especially at its beginning; the other begins from the ves- 

 tibulum. The cochlea is a spiral canal coiled within itself, and divided into 2 

 by a thin spiral bony plate, which is completed in the recent subject, and forms 

 2 perfect canals. In the recent subject, the foramen rotundum is lined with the 

 membrane of the tympanum, which terminates in a blind end, forming a kind 

 of membrana cochleae. The other opening, in the recent subject, communi- 

 cates with the spiral turn, beyond the membranous termination of the foramen 

 rotundum. 



The foramen ovale has a little projection inwards all round, on which the 

 stapes stands: within this is the vestibulum, which is common to the other spiral 

 turn of the cochleae, and the semicircular canals; this canal of the cochlea 

 passes out first in a direction contrary to its general course, but soon makes a 

 turn into the spiral. It is round, and not merely a division of the cochlea into 

 2 by a septum, but has a membrane of its own, which is attached to the thin 

 bony plate, and lines that part of the cochlea in such a manner as to retain its 

 structure when the bone is removed. The cochlea in some completes one turn 

 and a half; in others, more. It is not a spiral on a plane, or cylinder, but on 

 a cone. 



I have already observed, that by looking in at the foramen rotundum, we see 

 2 small ridges; the uppermost is the swell of the canal from the vestibulum just 

 described; the lower ridge, which is also a canal, may be observed just to pass 

 along the foramen belonging to this canal, close to the septum between the 2 ; a 

 circumstance I believe peculiar to this tribe. Its beginning is close to the ves- 

 tibulum, but does not open from it, and passes along the first described spiral 

 turn to its apex: when opened it appears to be a canal full of small perforations, 

 probably the passages of the branches from the auditory nerve. This bony 

 process has several perforations in it; one of them large, for the passage of the 

 7th pair of nerves. The size of the portio mollis, before its entrance into the 

 organ, is very large, and bears no proportion to that which enters. The passage 

 for this nerve is very wide, and seems to have an irregular blind conical, and 

 somewhat spiral, termination; its being spiral arises from the closeness to the 

 point of the cochlea. In the terminating part there are a number of perforations 

 into the cochlea, and one into the semicircular canals, which afibrd a passage to 

 the different divisions of the auditory nerve. There is a considerable foramen in 



