Fig. 59.—SHOWING THE DISTRIBUTION OF NERVES AND VEINS, AND ILLUSTRATING THE MANNER IN WHICH THEY PASS THROUGH THE FLESH TO REACH THE PARTS TO WHICH THEIR FUNCTIONS BELONG.
A. B. Veins of the fore-arm.
B. Canal formed in the muscle, through which a trunk-vein emerges.
C. Canal formed in the muscle, through which a large nerve emerges.
D. Canal through which a vein enters to communicate with the deep muscles of the arm.
956. What is the constitution of a nerve?
It consists of a thin membrane, or sheath, surrounding a greyish oily matter, which forms the nervous marrow. In the centre of this marrow is usually found a small fibre, which is supposed to be the essential part of the nerve; and most nerves consist of a number of these sheaths enclosing fibres running in parallel directions.
957. What is the nervous fluid?
The term nervous fluid is used to express our ideas of the mode by which the brain and spinal cord influence the remote parts: just as we say the electric fluid, without knowing that such a fluid exists. It is the most convenient form of expression.