372 INTERNAL EAR OR LABYRINTH. 



about one-twentietli at its summit. It makes a spiral coil 

 of two turns and a half around a central axis or modiolus, 

 and is compared to a winding stair-case. It is separated by 

 a partition, called lamina spiralis, into two tubes, named 

 scalce, (scala, a stair-case.) The lamina spiralis consists of 

 s an osseous and membranous portion. The osseous forms 

 the inner portion, being in contact with the modiolus, round 

 which it winds. It is broader at the base and gradually 

 diminishes to the apex, where it ends in a hook called the 

 hamulus. The membranous part of the lamina completes 

 the outer portion of the septum and is attached to the inner 

 surface of the tube of the cochlea, it is the converse of the 

 osseous plate, and is broader above than below, where it 

 forms the entire septum. 



Both the osseous and membranous portions of the lamina 

 spiralis consist of two thin plates, leaving a space, in which 

 the cochlear nerves and vessels are distributed. This 

 lamina spiralis or septum of the tube of the cochlea, divides 

 the tube into the scalce. 



The Scalce are two in number the one communicating 

 with the vestibule called scala vestibuli the other, with 

 the tympanum, named the scala tympani. These scalas are 

 separated throughout their whole extent, except at the 

 summit of the cochlea, over the hamulus, where they com- 

 municate by one common opening. The scala vestibuli is 

 external and superior while the scala tympani is internal 

 and inferior. 



The Modiolus constitutes the central axis, or pillar of the 

 cochlea, extending from its base to its apex, around which 

 both the tube and spiral lamina make their turns. It arises 

 from the bottom of the internal auditory meatus, as a bony 

 process, forming a conical tube which proceeds horizontally 

 outward, and contracts as it reaches the apex. The base of 

 the modiolus is perforated with foramina, for the passage 

 of the auditory nerves. Its funnel-shaped summit is called 

 the infundibulum, and is arched over,, or surmounted by the 

 blind apex of the tube of the cochlea, named the cupola. 

 The surface of the modiolus presents numerous canals 



