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Cells, again, preside over the chemical processes which elaborate the living fluids; they change their form to become the agents of voluntary and involuntary motion; the soul itself sits on a throne of nucleated cells, and flashes its mandates through skeins of glassy filaments which once were simple chains of vesicles.
But before we consider the development of the complicated structure of the brain from this simple series of vesicles, let us glance for a moment at the lower animals, which have no brain. Even in the skull-less vertebrate, the Amphioxus, we find no independent brain, as we have seen.
The specific differences lie below this general resemblance of outward form; we cannot discern them, but we know that they must exist, and that they are essential differences, for each one of these vesicles is invariably developed, if at all, into an individual of the species to which its parent belongs.
The disease may be accompanied by an eruption of nodules, or vesicles on the skin, or nasal mucous membrane. In severe and chronic cases multiple abscesses may form. This complication is indicated by emaciation and weakness. Such cases usually terminate in death. Severe inflammation and swelling in the region of the throat may terminate in strangulation and death.
It is unquestionable that the Limnophysalis hyalina enter into the blood either by the bronchial mucous membrane, by the surface of the pulmonary vesicles, or by the mucous membrane of the intestinal canal, most often, no doubt, by the last, with the ingested water; this introduction is aided by the force of suction and pressure, which facilitates their absorption.
Clovers and rape are not as dangerous a feed. The actual cause of the skin becoming inflamed is not known. The skin in the regions of the face, ears, neck, lower surface of the body and limbs becomes red and covered with vesicles. Later, scabs and pus may form. The treatment consists in changing the ration and keeping the animals out of the sun, or long grass and weeds for a few days.
Cases of too frequent nocturnal emissions accompanied by languor and headache are usually caused by irritability or lack of tonicity of the sexual apparatus, particularly of the seminal vesicles and the ducts.
According to the cell doctrine then formulated, the parts of all animals and plants are either composed of cells or of material derived from cells. The bark, the wood, the roots, the leaves of plants are all composed of little vesicles similar to those already described under the name of cells.
The back and chest may be rubbed from time to time with a stimulating liniment, and an emetic of ipecacuanha wine may be given twice a day. The act of vomiting not only removes any of the mucus which is apt to accumulate in the larger air tubes, but the powerful inspirations which follow the effort tend to introduce air into the smallest vesicles of the lungs, and to do away with their collapse.
The human brain, seen from below. But comparative anatomy and ontogeny teach us that in man and all the other Craniotes the brain is at first composed, not of these two, but of three, and afterwards five, consecutive parts. These are found in just the same form as five consecutive vesicles in the embryo of all the Craniotes, from the Cyclostoma and fishes to man.