ii 4 PRACTICAL HISTOLOGY. 



and part of the vestibule. Place the pieces at once in a mixture of three parts of Miiller's 

 fluid and one of spirit for two weeks. After that transfer them to a quarter per cent, solution 

 of chromic acid for a fortnight. At the end of that time add a few drops of hydrochloric acid 

 to the fluid until the bone is softened. The bone around the labyrinth is very dense, and 

 requires a longer time than the other parts to become soft. Wash it thoroughly for twelve 

 hours in water, and place the whole cochlea for thirty-six hours in syrup and forty-eight in 

 gum, and freeze it in gum in a freezing microtome. 



The student must examine a series of preparations prepared by the above methods. 



Vertical Section of a Cochlea. EXAMINATION (L). Observe the modiolus or central 

 column in the centre, with the cochlear branches of the auditory nerve ascending within it. 

 Note a section of one of the turns of the cochlea. Each turn is divided into two compartments 

 by the lamina spiralis. The upper compartment is called the scala vestibuli, and the lower 

 one the scala tympani. Note that the lamina spiralis consists of an inner osseous portion con- 

 sisting of two plates of bone between which runs a branch of the cochlear nerve on its way to 

 structures which rest on the outer or membranous part of the lamina. In the course of the 

 nerve observe the spiral ganglion. The membranous part consists of a basilar membrane, on 

 which rests the organ of Corti and certain epithelial cells which are connected with the ter- 

 minations of the cochlear branch of the auditory nerve. It is connected to the outer wall of 

 the cochlea by the spiral ligament, which consists of a mass of connective tissue, continuous 

 with the periosteum of the bone. 



Notice a thin membrane running obliquely upwards and outwards in the scala vestibuli, 

 from the outer part of the lamina spiral ossea to the outer wall of the cochlea ; this is the 

 membrane of Reissner, which cuts off a narrow triangular channel, the ductus cochlece, or scala 

 media, from the scala vestibuli proper. The floor of this narrow canal is made up of the outer part 

 of the osseous lamina and the basilar membrane. At the outer extremity of the osseous lamina 

 is a spiral groove excavated like a C, with the concavity looking outwards ; this is the sulcus 

 spiralis. The projecting upper edge of the groove is called the crista spiralis. Observe the 

 organ of Corti, consisting of an inner and an outer set of rods resting on the basilar membrane. 

 These two sets of rods incline towards each other, their heads touching, so as to leave a 

 triangular passage or tunnel under them, which is filled with endolymph. The outer rods are 

 longer than the inner ones. Internal to the inner rods and external to the outer ones are a 

 number of epithelial cells, some of them with hairs attached to them the outer and inner hair- 

 cells. A faintly-striated membrane springs from the upper surface of the lamina spiralis ossea, 

 close to the crista spiralis, and may be seen partially to cover the organ of Corti. It was 

 called by Claudius the membrana tectoria. It terminates over and is attached to the outer 

 hair-cells. The outer and inner hair-cells send prolongations through another membrane 

 which covers them the membrana reticularis. (Indicate the general arrangement in PL XXVII., 



F'g- 30 



(H). The membrane of Reissner consists of a very delicate hyaline-membrane covered on 

 each surface with a single layer of epithelium. Its surface, directed towards the cochlear canal, 

 is composed of polyhedral flattened cells, whilst the vestibular surface is covered by very thin 

 endothelial cells. The membrana basilaris has, on its upper surface, many epithelial cells, 

 some of them specially to form the organ of Corti, and others in which the cochlear branch of 

 the auditory nerve probably terminates, and which are, in fact, its ' end-organs.' The basilar 

 membrane consists of one or two layers of hyaline membrane, which are covered on the tym- 

 panic side by a single layer of endothelial cells. On the upper or vestibular surface of the 



