582 MR. youatt’s veterinary lectures. 
taken as the subjects of experiments, the same chemical agents 
act in precisely the same way on the pulpy matter of the brain and 
the nerves; yet, as the different parts of the brain seem to differ 
much in structure, the nerves present as striking differences. 
The proportion and density of the neurilema constitute, perhaps, 
the chief difference. There is not only the external theca, but 
every nerve, even the most pulpy, consists of numerous filaments, 
or distinct portions of nervous matter, each surrounded by its 
peculiar sheath. A thousand—ten thousand of these little threads 
may be bound together to constitute not only a compound, but 
a simple nerve. The manner in which they are thus bound may 
constitute a very great difference in the appearance and function 
of the nerve. In the olfactory nerve the threads may be large, 
and the thecse of gossamer structure. In the optic nerve the 
filaments have a tubular shape, with a little canal or hollow in 
the centre. In the pneumogastric nerve there appears to be a 
single chord only; but it is divided by numerous furrows. In 
some branches of the ganglial, the filament is surrounded by a 
net-work or plexus of smaller ones. In the fifth pair many of 
the filaments are not only large, but the theca is unusually dense, 
and conjoined with these are numerous smaller filaments. 
Bloodvessels o f the Nerves ,—Every nerve is attended by its 
corresponding artery, usually derived from the nearest arterial 
trunk. It is mostly seen on the theca, forming upon it the most 
complicated plexus of vessels. The capillaries penetrating into 
the medullary substance, usually carry only a colourless fluid; 
but they are capable of enlargement, depending on the frequently 
important pathological distinction, whether it is the neurilema 
or the nervous pulp that is labouring under inflammatory action. 
The veins are as readily traced ; but, as in the brain, they do not 
usually follow the course of the arteries, nor even penetrate 
through the neurilema at the same point. Absorbents must ne¬ 
cessarily accompany the nerves in their course ; but no dissection 
or injection, however minute, has yet demonstrated them. 
I must not say more until we actually examine the structure, 
and trace the course, and determine the function of the cerebral 
nerves ; but these somewhat unconnected, and possibly not very 
interesting, observations, were necessary to prepare us for that 
examination which will be attempted in the next lecture. 
