It is the introduction of the middle column of the three, viz. that for respiration, which constitutes the spinal marrow, as distinct from the long central nerve of the animals without vertebræ, and which is attended with the necessity for that form of the trunk which admits of the respiratory motions.

In animals which do not breathe by a uniform and general motion of their bodies, there is no spinal marrow, but only a long compound and ganglionic nerve, extending through the body for the purpose of sensation and motion. This cord in those creatures does not actuate the animal machine with alternate dilatation and contraction. There may be a motion of some part which admits and expels air from a cavity, or agitates the water, and which motion is subservient to oxygenation of

the blood; and there may be a nerve supplied to that apparatus with sensibility and power suited to the function thus to be performed, and resembling our par vagum in office; but there is no regular and corresponding distribution of a respiratory system of nerves to both sides of the body, and no arrangement of bones and muscles, for a general and regular motion of the frame like that which takes place in vertebral animals, and which is necessary to their mode of existence.

25

OF THE NERVES WHICH ARISE FROM THE SPINAL MARROW.-COMPARISON WITH THE NERVES OF THE ENCEPHALON.

THE first conception which I entertained of the true arrangement of the nerves, arose from a comparison of the nerves which take their origin from the brain, with those which arise from the spinal marrow. The perfect regularity of the latter, contrasted with the very great irregularity of the former, naturally led to an enquiry into the cause of this difference. I said, if the endowment of a nerve depend on the relation of its roots to the columns of the spinal marrow and base of the brain, then must the observation of their roots indicate to us their true distinctions and their different uses.

The spinal nerves are perfectly regular in origin and distribution, and are thirty on each side.* Each nerve has two distinct series of roots coming out in packets or fasces, one from the posterior column, and one from the anterior column, of the spinal marrow.

* The tenth nerve of the head, as enumerated by Willis, and called suboccipital from its situation, is in

The posterior fascis is formed of funiculi, which come out with remarkable abruptness from the column; and their roots form a very regular row or series along the sides of the spinal marrow. They seem at once to burst out from the confinement of the arachnoid coat. These funiculi, converging towards the foramen of the sheath of the spinal marrow, and being collected together, form a ganglion. This ganglion is not seen within the sheath of the spinal marrow; its seat is in the part where the fascis is surrounded and united to the sheath, and just before this root of the nerve joins the anterior one to constitute a spinal nerve.

constitution a spinal nerve; i. e. it has a double root or ganglion on its posterior root, and its distribution is similar to the spinal nerves, quite unlike those of the encephalon