M'DoNNELL — On the Nervous System. 
45 
the naked eye, or by the aid of the microscope, they m-ust have pre- 
existed in the solid form, and been enclosed while in a state of suspen- 
sion. Of the latter kind are the rhombohedrons of calcite, containing 
a large quantity of sand, found at Tontainbleau ; and the prisms of 
quartz rock, which consist of quartz sand cemented by orthoclase, 
meta-silicate of zinc and quartz enclosing silicate of iron, calcite, 
and chalybite enclosing clay, &c. Many dark-coloured hornblendes 
contain amorphous endomorphs, which look like enclosed suspended 
matter ; but we have no means of determining its character. It is 
probable, however, that, in some instances, at least, it is a silicate of 
iron, for we find some of them associated with crystals of Lievrite. 
In conclusion, I think that the opinion of Kenngott and Frankenheim, 
that the alumina of aluminous hornblendes and augites is present as an 
impurity, is true ; and further, that it is not present in the form of one 
or more specific silicates peculiar to them, but chiefly as a felspathic 
mineral, and derived from the aqueous or igneous mass out of which 
they have crystallized. Again, that, sometimes, in addition to a feld- 
spar, garnet, olivine, &c., some of the alumina is often present as an 
aluminate — chiefly as aluminate of magnesium or spinel. And, lastly, 
that the endomorphs and paragenitic minerals are indications of the 
heteromorphic chemical molecules which most likely enter into the 
composition of the constituent crystalline molecules, of all varieties, 
which deviate from the normal types of a group. 
XI. — On a ITew Theory of JSTeevous Action as regards the Trans- 
mission OF Sensation along the Nerves. Ey Robert M'Donnell, 
M. D., F. E. S. [Abstract.] 
[Read May 23, 1870.] 
A LARGE number of facts have of late years been observed, tending to 
show that what has hitherto been regarded as the sense of touch is ca- 
pable of being resolved into a number of comparatively elementary sen- 
sations, as those of temperature, contact, tickling, pain, &c. 
Many cases have likewise been observed in which some of these 
sensations are felt, while others cease to be perceived by the patient. 
Thus the individual may feel perfectly the contact of the hand, when 
lightly rubbed over the surface, yet not be able to distingaish heat from 
cold, or vice versa. 
Analogous phenomena are observed with regard to the other senses, 
as in cases of colour blindness, absence of ear, or inability to hear par- 
ticular notes, &c. 
In explanation of these and other kindred phenomena, it has been 
supposed that there exist in every nerve groups of distinct conductors, 
each adapted to convey along it distinct nervous impressions. 
This hypothesis is indeed that which is at the present time adopted 
