The proper quality and chemical composition of the lymph, which is different from that of the blood, is of no less importance than that of the plasma for the preservation and regeneration of the organism.
What the plasma is to the blood, the lymph is to the nerves.
3. The nerve tissue: A particular aggregation of cells forms the nerves, which, emanating from their center in the brain and spine, run as another separate system all through the body.
This system, however, is not one of vessels; but the nerves may best be compared to the wires of a telephone system, establishing connection between the remotest parts of the body and its central point, from which the directions for both voluntary and involuntary movement are given and transmitted through the nerves.
They are of a peculiar chemical composition in which the nerve fat (lecithin) plays a very important part, since its frequent presence in insufficient quantity is among the most common causes of a great number of nervous and other diseases.
4. The bone tissue: The bones consist of a special and very distinct tissue in which lime predominates. This gives them the strength and solidity which enables them to act as support to all the other organs.
The bones too are fed by the blood, and it is through the blood that the necessary constituent parts for the regeneration of their tissue is conveyed to them.
While naturally their power of resistance is greater than that of any other organ, they are nevertheless subject to a number of structural disturbances, other than traumatic, the causes of which are sometimes hereditary, sometimes acquired through deficient properties of the nourishing blood.
Certain tissues which form the connection between the bones and the rest of the organs, and the gradual transition into other tissues, are subjects separate and distinct and will be treated separately.
5. The muscular tissue: As to quantity, the muscular tissue represents the maximum of any in the human body.
The muscles do not only consist solely of this one tissue, but of several others, as do most of the other organs; but here, as in all other cases, the principal component element is called after the organ in which it is chiefly found.
The structure of the muscular tissue varies according to its function, so that we distinguish between the striated and the unstriated or smooth muscles. This, however, has no influence on their chemical composition, a distinctive element of which is muscular fibrin, which has the particular property of contractibility.
6. The mucous membrane tissue: The mucous membrane forms the covering of many of the organs, and its chemical and structural composition is identical in all parts of the body.
It is characterized by a viscid watery secretion from the mucous glands, which are always found in the mucous membrane.