III. MEASUREMENT. 937 



skull ; two pieces of steel plate are cut so as to fit into the saw-draughts, and 

 the poshion in which they fit indicated by marks on the plates and on the 

 mastoid bone. The greater part of the mastoid and the whole of the petrous 

 portion are then sawn out, and the steel plates fixed to the portion of bone 

 thus removed. This portion can now be replaced exactly in situ by placing 

 the ends of the steel plates into the portions of the saw-draughts remaining on 

 the skull. The remaining portion of bone is now plunged into a bath of 

 fusible metal, and placed under the receiver of an air pump. On exhausting, 

 bubbles of air escape from the cavities in the bone, and, on re-admitting air, 

 these cavities are filled with fusible metal. By repeating this operation ten or 

 twelve times in different positions, all the air can be pumped out and the 

 cavities completely filled with fusible metal. The bone is now removed from 

 the bath of fusible metal, and the adherent metal removed. It is then placed 

 in melted paraffin, so as to cover the steel plates, and the greater part of the 

 mastoid portion. When the paraffin has solidified, the whole is placed in a 

 vessel containing dilute hydrochloric acid. This dissolves the unprotected 

 part of the bone, leaving a cast of the cavities in fusible metal. From this 

 the casts of the mastoid cells are 'carefully removed. To the external part of the 

 mastoid is now soldered a brass pin fitting into a socket in the large gonio- 

 meter. The cast can then be brought successively into positions in which 

 each of the canals lies in a horizontal plane. In each of these positions the 

 skull is replaced by means of the steel plates, and a glass plate fixed to the 

 skull in a horizontal plane. These plates are therefore parallel respectively 

 to the planes of the canals ; and their relative position can be ascertained by 

 means of the goniometer. See Proceedings of the Koyal Society of Edinburgh, 

 January 1874, and Journal of Anatomy and Physiology, viii. 327. 



3734. Skull of Owl, prepared to show relative position of the 

 semicircular canals of internal ear. 



The spongy osseous tissue has been removed from the dense bones forming- 

 the canals, and these have been coloured so as to indicate the pairs of parallel 

 canals. 



3735. Skull of Crow, prepared to show relative position of 

 the semicircular canals of internal ear. 



3736. Skull of Heron, prepared to show relative position of 

 the semicircular canals of internal ear. 



3737. Cast, in plaster of Paris, of internal ears of the skate. 



3738. Apparatus for measuring the relative position of the 

 planes in which the semicircular canals of the internal ear 



are situated, with skull, illustrating its application. 



The apparatus is simply a reflecting goniometer on a large scale. The 

 telescope used with it is not sent, as it requires to be fixed at a considerable 

 distance from the apparatus. 



The mode of preparing the ear for observation is described above. 



3739. Cast, in solder, of human internal ear, right side. 



3722. Craniometer, by which measurements of crania may be 

 taken rapidly and accurately. Prof. Struthers. 



It is essentially a glass box, the panes accurately ruled and fitted into a 

 carefully made brass frame. Sides 9 inches square ; ends, bottom, and top, 



