General Anatomy, Applied to Physiology and Medicine, Vol. 1 (of 3)

by Xavier Bichat

I. Texture peculiar to this organization.

This organization is nearly the same for the whole system, in the common membrane that forms the great canal in which the black blood is contained; but it differs in the textures that are connected exteriorly with this membrane. In the heart the texture is fleshy; it is analogous to the texture of the divisions of the aorta, in the pulmonary artery; it has a peculiar character in the veins; it is this that will now particularly engage our attention.

Membrane peculiar to the veins.

In order to see this membrane, it is necessary to remove, 1st, the loose cellular texture that unites the veins to the neighbouring parts; 2d, the cellular layer of a peculiar nature that immediately covers them, and of which we have spoken in the article upon the cellular system. Then we distinguish in the great trunks, longitudinal fibres all parallel to each other, forming a very fine layer, often difficult to be seen at first view, but always having a real existence. When the veins are much dilated, these fibres being more separated, are less evident than in a state of contraction. These longitudinal fibres are seen more clearly in the trunk of the inferior vena cava than in that of the superior. In general it may be said, that they are also more marked in all the divisions of the first than in those of the second; dissection has convinced me of this. Undoubtedly this arises from the greater facility that the blood experiences in circulating in the second than in the first of these veins, in which it mounts against its own weight; this is moreover a proof that man was designed to go erect.

I have uniformly made another remark, it is, that in the superficial veins these fibres are much more evident than in the deep-seated ones; the internal saphena is a remarkable example of this. It suffices to open it in its course, to see very distinctly its fibres through the common membrane, especially if it is a little contracted. By cutting the crural vein to compare it with this, it is easy to perceive the difference, which arises without doubt from this circumstance, that the neighbouring parts assist the circulation in the deep veins, whilst there is less of this assistance given to the superficial ones.

The branches have fibres in proportion greater than the trunks; hence the proportional excess of the thickness of their parietes, their greater resistance to the blood, their less frequent dilatation, &c.

At the place where any branch arises from a trunk, we observe that the fibres change their direction and go upon the branch, a circumstance that distinguishes them from arterial branches, whose fibres are not a continuation of those of the trunk.

Venous fibres oftentimes approach each other, are united, and give a greater thickness to the vein; this is frequently observed at the origin of the saphena. I have also seen this arrangement in the hypogastric vein; Boyer has likewise noticed it.

In general, the venous fibre, except in these places, is remarkable for its delicacy, for the little thickness that it consequently gives to the membrane that it forms. The peculiar membrane of the arteries infinitely exceeds that of the veins in this respect; it is this delicacy that favours to a great degree venous extensibility. Observe that the structure of each kind of vessels is adapted to their peculiar circulation. If the blood circulated in the veins with parietes like those of the arteries, its motion would be continually disturbed. A thousand causes retard the venous blood; when its motion is languid, the capacity of the vessels is increased; now, the arterial textures not allowing of this dilatation, it is evident that the circulation would be interrupted. If then the agent of impulse, placed at the commencement of the arteries, requires a firm and unyielding texture, the slow motion of the blood in the veins, the frequent causes that retard its progress, demand a texture of an opposite character.

What is the nature of the venous fibre? Its appearance, its want of elasticity and brittleness, its great extensibility of texture, its softness, its colour, its direction, distinguish it completely from the arterial fibre. Is it muscular? it does not appear to be irritable, and it does not look like the muscular fibres. I believe that it is of a peculiar nature, essentially different from that of all the other textures, having its peculiar properties, life and organization; I do not think it capable of much motion. We have, however, but few data upon this point.