Each nerve receives its vessels from the surrounding trunks, which send to it branches which penetrate from all sides to the interior. The optic is an exception to this rule; the membrane that surrounds it prevents the vessels from entering it laterally. An artery passes through it in the course of its axis and sends out different branches.
In the other nerves, the arteries run first in the cellular texture between the cords, and are of a size there more or less considerable, according to the nervous trunks. Sometimes this size increases considerably. For example, in popliteal aneurism, the artery of the sciatic nerve has been seen with a caliber more than three times its natural size.
The arteries running between the cords, send off a number of little branches that go into all the interstices of the filaments. In fine, from these arise the little capillary arteries which are spread upon the coat of the nerve, cross it and are continued with the exhalants of the medullary substance. We readily see this vascular arrangement in the spinal marrow. Numerous ramifications are spread first upon the dense and firm pia mater, which there takes the place of the nervous coat; they then penetrate the medullary substance, and are lost in continuation with the exhalants.
The veins follow in the nerves a course analogous to the arteries; however, in carefully dissecting many great nervous trunks, I have been convinced that their branches do not go out of the nerves at the place where the arteries enter. This arrangement is analogous to that of the brain, where the arteries enter below and the veins go out above.
Many authors, particularly Reil, have overrated the quantity of blood that goes to the nerves, because, in order to ascertain it, they have made use of fine injections which have entered the capillary system, which does not commonly contain red blood. I am convinced by dissecting the nerves of living animals, the only means of having an accurate idea of what takes place in a natural state, how uncertain this method is here as every where else.
The blood that goes to the nerves, like that which is sent to the brain, is a stimulant that supports their action. When this stimulant is increased the nervous excitability increases, as Reil was convinced by rubbing the nerves of a frog, so as to redden them by the quantity of blood that was brought there. Does this fluid, when carried in great quantity to the nervous system, sometimes interrupt its functions, as happens to the brain in sanguineous apoplexy? I have not had an opportunity yet of making this observation in a very decided manner in the great number of bodies that I have opened. Only the nerves are a little more reddish in some cases than others. Do these cases coincide with certain determinate diseases? I have not any knowledge upon this point. As to the pretended compression of the origin of the nerves, by the blood that is carried to the brain and spinal marrow, whoever has examined the relations of the nerves with the vessels of the base of the cranium, will see that such a compression is not probable. Besides, most of the holes through which the little arteries penetrate into the interior even of this viscus, have a caliber greater than that of the arteries; so that how full soever they may be, they cannot press upon their parietes. I can only conceive of a compression at the origin of the nerves, from effusions at the base of the cranium.
Exhalants and absorbents.
We cannot discover these vessels in the nerves; but nutrition supposes their existence there. It appears that this function is performed in the following manner; the exhalants receive from the arteries, with which they are continuous, the medullary substance which they deposit in the canal of the nervous coat, which is, if I may so express myself the reservoir of this substance, that is afterwards taken up by the absorbents.
Many think that the nervous coat is the secretory organ of this medullary substance, which oozes out of its parietes, to lie in its cavity. I do not believe it, 1st. because the olfactory nerve would not then be able to support itself, any more than the posterior portion of the optic. 2d. The cerebral membranes are not concerned with the secretion of the pulp of the brain, they only permit the vessels to pass, which enter this organ to deposit it there. 3d. There is the same arrangement at the spinal marrow, the pia mater of which has so great an analogy with the nervous coat. The vessels cross this membrane, then losing themselves, as I have said, in the medullary substance constantly renew it; so that if it was possible to remove this substance without touching the vessels, these would hang loose by their extremities in the canal of the pia mater. Thus, in some very soft fungi, the vessels cross here and there the substance that they deposit in their interstices, and would produce a vegetation like net-work, if we could remove this substance and leave them untouched. 4th. In the optic nerve the vessels evidently are not confined to the nervous coat; they penetrate also the canals it forms, and deposit there the medullary substance.
Every thing appears then to prove, that the nervous coat is no more the secretory organ of the nervous substance, than the pia mater is of the cerebral substances or that of the spinal marrow. It may have uses of which we are ignorant; but the principal certainly is that of a covering; it is the passive part of the nerve, the medulla being the part essentially active.