Frontispiece · Ordo Custodia

The plates

The seven specimens

Seven plates: two kinds of vessel that drain the tissues, and five places where lymphocytes are made, trained or stationed. Two relatives live in other volumes. The tonsils and Peyer’s patches, the appendix’s partners in the wall of the gut, are on Ordo Alimentaria, Pl. XI. The fine net that holds a lymph node and the spleen together is reticular connective tissue, on Ordo Conjunctiva, Pl. III.

Pl. I

Hauriens caecus var. utriculariae

the trapdoor · lymphatic capillary

Open Pl. I →

Pl. II

Hauriens moniliformis var. hormosirae

the string of beads · collecting lymphatic vessel

Open Pl. II →

Pl. III

Colans lymphae var. hydropsychae

the net in the current · lymph node

Open Pl. III →

Pl. IV

Colans sanguinis var. coccinellae

the spotted coat · spleen

Open Pl. IV →

Pl. V

Erudiens lobulatus var. ascidiae

the school that closes · thymus

Open Pl. V →

Pl. VI

Gignens medullaris var. termitis

the nursery in the wall · red bone marrow

Open Pl. VI →

Pl. VII

Vigilans appendicis var. abelmoschi

the dead end · appendix

Open Pl. VII →

The ground plan

Cells that wait, and the fluid that is brought to them

The lymphatic system is two things working together. One is plumbing: thin vessels that collect the fluid blood capillaries leave behind in the tissues and carry it back to the veins. The other is a population of lymphocytes, small round cells that are nearly all nucleus, parked at fixed stations along the way and waiting for something to recognise. Where they gather thickly they form a follicle, and the follicle is the unit this whole volume is built from.

Fig. 1 · the follicle, at rest and at work

A dark ball is resting. A dark ring round a pale centre has met something.

The five words

• Lymph — tissue fluid, once it is inside a lymphatic vessel. Pale, watery, and carrying a few lymphocytes.
• Lymphocyte — a small round cell, nearly all nucleus. Packed together they make a tissue look dark purple.
• Follicle (or nodule) — a round, crowded ball of lymphocytes, mostly B cells.
• Germinal centre — the pale middle a follicle develops when its B cells have met an antigen and begun dividing.
• Capsule — a coat of dense connective tissue round an organ. Some of these have one and some pointedly do not.

So before anything else, find the small dark nuclei, and ask whether they are gathered into follicles.

First question

Is it a tube, or is it a mass?

Two of the seven are vessels. They are easy to walk past, because a lymphatic looks like an empty vein.

Fig. 2 · three thin tubes

Look at what is inside before you look at the wall.

• A thin-walled channel that begins at a closed end, with overlapping cells and nothing red inside — a lymphatic capillary: Pl. I.
• A thin-walled, beaded vessel with a valve at every pinch and pale contents — a collecting lymphatic: Pl. II.
• Red cells inside — a blood vessel, however thin its wall. It belongs to Ordo Vasa, not here.
• And if it is not a tube at all but a mass of small dark nuclei, go on to the second question.

This one question names two of the seven, and it is settled by the contents: pale and nearly empty is lymph, red is blood.

Second question

A mass of lymphocytes: which one?

Three checks, in any order: is there a capsule, are there follicles, and what is in the spaces between?

Fig. 3 · four masses of lymphocytes, at low power

Capsule, follicles, spaces. Three answers name each one.

• A capsule and follicles. Follicles at the rim with pale lymph-filled sinuses inside — a lymph node: Pl. III. Follicles round small arteries in a red, blood-filled field — the spleen: Pl. IV.
• A capsule and lobules, each dark at the rim and pale at the core, and no follicles at all — the thymus: Pl. V.
• No capsule. Follicles sitting directly under an epithelium, in the wall of a tube — MALT. In a complete ring round a small lumen it is the appendix: Pl. VII. For tonsils and Peyer’s patches see Ordo Alimentaria, Pl. XI. And fat cells with every sort of cell mixed between them, inside bone, is red marrow: Pl. VI.

Notice what the follicle tells you. Present, and where: node, spleen, MALT. Absent: thymus, where cells are trained and nothing is fought.

The naming key

How to read a name

Genus is the job the specimen does. Species is the mark you find it by, or what it works on. The var. is the organism on the plate.

Worked both ways

• Colans sanguinis → the one that strains blood → the spleen.
• Hauriens caecus → a drain with a blind end → a lymphatic capillary.
• You saw lobules, each dark outside and pale inside, and a pink whorl in one pale core → Erudiens lobulatus → Pl. V.

Two that trip people up

The look-alikes

Two mistakes account for most of the lost points in this unit. In both, the overall colour is the same on each side, and something small decides it.

Fig. 4 · two pairs that trip people up

A corpuscle or a follicle settles the first. The contents settle the second.

Keying out a specimen

Three questions, in this order

This is the procedure. Work down it, and do not skip to the one you think you recognise.

1. A tube, or a mass? A thin-walled tube with nothing red inside is a lymphatic. Closed at one end with overlapping cells → capillary. Beaded, with valves → collecting vessel.
2. A mass of small dark nuclei: is there a capsule, and are there follicles? Capsule and follicles → lymph node or spleen; go to step 3. Capsule, lobules, no follicles → thymus. No capsule, follicles under an epithelium → MALT; a full ring round a small lumen is the appendix.
3. What is in the spaces? Pale lymph, in a sinus under the capsule and through the middle → lymph node. Red cells everywhere, with the follicles as dark islands round small arteries → spleen.

Three overrides. Fat cells with a crowd of mixed cells between them, inside bone, is red marrow, whatever else you think you see. A Hassall’s corpuscle means thymus even when most of the slide is fat. And red cells in the lumen mean it is a blood vessel, however thin the wall.

YOur missions

Field Notes for Future Missions

Before you go to the plates, try the ground plan on a field you have not met yet.

Field notes

Why seven organisms

A bladderwort, a seaweed, a caddisfly, a ladybird, a sea squirt, a termite mound and an okra pod have nothing to do with immunity. That is the point. Each one already shows, out in the world, the one structural idea its specimen turns on: a door that only opens inward, a chain of beads, a net set across a current, dark islands in a bright field, a job finished in youth and then dismantled, many kinds from one source inside a hard wall, and a ring of round bodies in the wall of a dead end.

This system is easy to treat as a list of organs that all look like purple dots. They are not alike. Each one is in a different place for a different reason, and on a slide each gives itself away by where its follicles are, by whether it has any, and by what fills the spaces between.