At the close of this compressed survey of the order Coniferæ, let me put the chief forms and groups at which we have looked in a tabular form, thus:
Order CONIFERÆ:
| Family 1.—ABIETINÆ. |
| Section A.—Sapineæ (Spruces and Silver Firs). |
| Genus 1.—Picea. 2. Tsuga. 3. Pseudotsuga. 4. Abies. |
| Section B.—Lariceæ (Larches and Cedars). |
| Genus 1.—Larix. 2. Cedrus. |
| Section C.—Pineæ. |
| Genus unic.—Pinus. |
| Family 2.—ARAUCARIANÆ. |
| Genus 1.—Araucaria. 2. Agathis. 2. Cunninghamia. |
| Family 3.—TAXODINÆ. |
| Genus 1.—Sequoia. 2 Taxodium. 3. Sciadopitys, etc. |
| Family 4.—CUPRESSINÆ. |
| Genus 1.—Cupressus. 2. Thuya. 3. Juniperus, etc. |
CHAPTER XXXIII
THE LYMPH AND THE LYMPHATIC SYSTEM
MOST people do not know even of the existence in their own bodies of a fluid called "the lymph," and of a system of vessels and spaces containing it which ramify like the blood-vessels into every part of the body. This arises from the fact that the lymph is translucent and colourless. You can see the finest blood-vessels when the body of a dead rat, sheep, or man is opened, because they are filled with the beautiful red blood, and appear as a rich, coloured network. But the lymph and the lymph-vessels escape notice, and, indeed, are invisible except the largest, because they are colourless. They remained unknown to anatomists long after arteries and veins, and the fine networks of hair-like vessels or capillaries connecting them, were thoroughly well studied. It is, when one thinks of it, a very noteworthy fact, tending to convince us of the readiness with which we may (in the absence of careful examination and attention) overlook the most weighty things, that here is a great system of vessels and spaces in the human body and in that of other animals, carrying on most important operations in our daily life, and yet most of us have never seen any evidence of its existence, and never hold it in our mind's eye as part of the great mechanism of the animal body.
The lymph is a clear, colourless fluid, with "corpuscles"—minute nucleated cells or particles of protoplasm—floating in it. The liquid part is closely similar in its properties and chemical constitution to the liquid part of the blood. It, indeed, consists largely of the liquid part of the blood which exudes from the finest hair-like blood-vessels or capillaries as they traverse the various tissues, and it is the chief business of the "lymphatics" or lymph-holding vessels to return this exuded liquid to the blood system, which they do by joining—like the rivulets of a river system—to form two large trunks which open into the great blood-holding veins at the region where they approach the heart. The total amount of lymph in the lymphatic system is difficult to estimate, but it is larger in quantity than the blood in the entire blood-vascular system. A large number of the delicate vessels of the lymphatic system take their origin just below the lining layer of the intestine, and ramify through the transparent membrane, which holds the coils of intestine together, and is called the mesentery. The fatty or oily materials of food pass through the lining "cells" of the intestinal wall into these "lacteal" or milky lymphatics, and consequently in an animal killed and examined after a meal, the fluid in them has a milky appearance, and renders this kind of "lymphatics" visible.
They were for this reason the first to be detected, and were known even in ancient times to anatomists. The milky fluid in them was called "the chyle." Its milky appearance is due to the same cause as the white opaque appearance of milk, namely, to the presence of an immense number of excessively small particles of oil (fat) and a certain proportion of larger globules of the same nature. It was thus not difficult for the old anatomists to trace the fine branches of the lacteals uniting branch to branch, and at last forming a large trunk—called the thoracic duct—about a quarter of an inch thick, which runs up the inner face of the backbone to the neck, where it joins the great left subclavian vein, and pours its contents into the blood-stream which is there nearing the heart. A small trunk formed by the union of lymphatic vessels from the right side of the head and neck and the right upper limb opens into the right subclavian vein. It took some time to discover this smaller trunk, since it is not brought to view by milky contents. Gradually it was made out that there are innumerable transparent branches opening into the thoracic duct from the whole of the body, besides the milky-looking lacteals: branches which bring "limpid" clear fluid, or "lymph," from all the viscera, from the muscles, and from the deeper layers of the skin in every region of the body, even from the toes, fingers, and tongue tip. In fact, wherever the blood-vessels take blood there are also vessels of the lymphatic system bringing back to the heart the liquid exudation which escapes into the tissues from the finest blood-vessels (Fig. 43).