72 JOURNAL OF COMPARATIVE NEUROLOGY. 
It includes a review of the anatomical features of the selachian 
nervous system ; a description of the structural elements com- 
mon to all nerve-centres ; topographic histology, or the struc- 
ture of the several parts of the brain and cord; and, finally, a 
consideration of the homologies of the brain-segments. The 
figures which accompany the paper are perfectly clear in their 
execution, but they represent such a low degree of magnifica- 
tion that they are really little more than diagrams. While the 
observations recorded by this author are of the most general 
character, he should receive great credit for interpreting the 
brain-segments properly at a time when there was much con- 
fusion in this respect. 
In the prosecution of his research, Rouon ('77) had all 
the stimulating advantages of the laboratory of Ciaus, and his 
work has a high order of merit. There is a section devoted to 
the comparative anatomy of the cranial nerves and the several 
regions of the brain, in both the rays and the sharks. Certain 
figures illustrating this portion of the work are familiar to all 
comparative anatomists through their reproduction in the text- 
books. The histological portion of the research is carefully 
written, and it is clear that the writer had seen all that the 
technique of the period would demonstrate. We find in his 
figures, therefore, nerve-cells represented with some detail of 
structure, nerve-fibres showing some connection with par- 
ticular groups of nerve-cells, and fibre-tracts which take a 
certain definiteness in their courses. The most noteworthy 
discovery made by Ronon was the Dachkerne of the midbrain ; 
see Section VI. But the greatest service which he has rendered 
consisted in his pointing out for the first time the many struct- 
ural features which the brain of the selachian has in common 
with the organization of higher vertebrate brains. 
The next research which we have to notice is that of SAn- 
DERS (86). This author seems not to have been familiar with 
the great advances just made in methods of research, (see infra); 
and so we find him rejecting carmine as a staining medium and 
expressing a preference for rosaniline because of the clearness 
of the pictures yielded by it, while he gives a mere hint of 
