I 3 4 PRACTICAL HISTOLOGY. 



e. Branched pigment cells of the substance of the 

 choroid. These may be seen in any thin section of the 

 tunics of the eyeball. 



COCHLEA. 



241. Methods. a. The structure of Corti's organ may be in- 

 vestigated in the fresh condition, but, except in very young animals, 

 little can be learned by such a mode of examination regarding the 

 relations of the various soft tissues. For this purpose, Corti's organ 

 must be hardened, its bony surroundings softened, and the cochlea 

 sliced vertically. It is of the greatest importance that the organ be 

 placed in preservative fluid as soon after death as possible. If the 

 human cochlea be prepared, it should be taken not later than twelve 

 hours after death ; and in order to preserve it as thoroughly as possible, 

 it is well, immediately after death, to inject rectified spirit into the 

 tympanic cavity with an ordinary subcutaneous syringe thrust through 

 the membrana tympani. 



b. Hardening. Osmic acid and chromic acid are the best hardening 

 agents. The latter is to be preferred when sections are to be made of 

 the surrounding bone. The best chromic fluid is the mixture of chromic 

 acid and spirit ( 4, Solution 3). This fluid is used in the manner 

 indicated in 17. 



c. Softening the bone. In the case of fcetal cochleae, the bone may 

 be softened by chromic acid alone, but for older cochleae a stronger 

 acid should be added. German authors recommend for this purpose 

 hydrochloric acid diluted with water ten times, but far better results 

 are obtained by using the bone-softening fluid already mentioned. 

 (See 4, Solution 5.) A large quantity of this fluid is necessary, and 

 in the case of the larger cochleae, they should be kept in motion by 

 some such means as an ordinary meat-jack. With the aid of this 

 device, even a large cochlea may be softened in a few days. 



d. Section. When sections are to be made of the softened cochlea, 

 the lamina spiralis and organ of Corti require support to maintain them 

 in situ. Melted wax and a hot solution of gelatine have both been 

 used for this purpose with some success, but the method of imbedding 

 in gum is decidedly preferable. Remove the cochlea from the softening 

 fluid, and place it in a small cone of bibulous paper, containing a strong 

 solution of gum arabic for four or five hours, then immerse the cone in 

 methylated spirit for forty-eight hours or so, until the gum has become 

 hard and tough ; then remove the superfluous gum, and imbed the 

 whole in the ordinary paraffin mixture in a microtome ( 303), with the 

 modiolus of the cochlea placed at right angles to the axis of the tube 

 of the microtome, so that Corti's organ will be sliced vertically. Or 

 transfer the cochlea from the softening fluid to a solution of gum placed 

 in the well of the freezing microtome, and, after letting it soak in the 

 gum for some hours, freeze and make sections. After either the one or 



