678 BRITISH ASSOCIATION FOR ADVANCEMENT OF SCIENCE. 
PHYSIOLOGICAL SECTION. 
President — Sir B. Brodie. 
On the Structure of the Choroid Coat of the 'Eye, and more particularly on the 
Character and Arrangement of the Pigmentary Matter. 
By Mr. Nunneley. 
The choroid coat is the dark tissue interposed between the delicate 
sentient retina and the hard, dense sclerotic, and coextensive with the 
latter. It begins at the entrance of the optic nerve by a round aperture, 
with a distinct edge, in close apposition with the nerve, but not 
organically connected with it, and passing forward as far as the junction 
of the sclerotic and cornea, where, as choroid proper, it terminates. It 
there comes in connexion with the ciliary circle or muscle, the ciliary 
body and the iris. The choroid is essentially a vascular membrane, 
being made up of blood-vessels, colouring matter, and a modified white 
fibrous tissue. The choroid universally provided the pigmentary nigrum, 
and is of a deep bronze colour, approaching to black. The pigment 
was described as consisting of two distinct forms of cells — on the inner 
surface the choroid, of true hexagonal cells, and in tlie tissue and on 
the posterior surface of stellate cells. The use of these cells was to 
destroy the light as soon as it had acted on the retina ; and they were 
the most perfect absorbers of light of any substance in nature that lie 
knew of. From the account he gave of the arrangement of the pig- 
ment, it afforded what he considered a satisfactory anatomical explana- 
tion of an abnormal condition of the eye which had hitherto not been 
understood, viz. Muscce volitantes. The figures of those motes he 
believed to resemble exactly portions of the choroid coat when teazed 
out ; and they might be expected to appear and disappear with the 
varying condition of the vessels arising from disordered stomach or the 
cerebral circulation, and be cured by whatever corrects those con- 
ditions ; or the muscae might result from different organic changes in 
the choroid coat, which are incapable of being removed. 
GEOLOGICAL SECTION. 
President — W. Hopkins. 
On the Introduction of Mammals on the Earth. 
“The President said the existence of mammalian life in its earlier 
stages on the surface of our planet, the condition of its existence, and 
the period of its introduction, have always furnished questions of 
the highest philosophical as well as palaeontological interest. You will 
be aware that some geologists regard each new discovery of mammalian 
remains, in formations preceding the older tertiaries, as a fresh indi- 
cation of the probable existence of mammalia in those earlier periods 
in which no positive proof of their existence has yet been obtained; 
while others regard such discoveries only as leading us to an ultimate 
limit, which will hereafter define a period of the introduction of mam- 
malia on the surface of the earth, long posterior to that of the first 
introduction of animal life. Be this as it may, every new discovery of 
the former existence of this highest class of animals must be a matter 
of great geological interest. An important discovery of this kind has 
recently been made, principally by the persevering exertions of Mr. 
