5. Living animal bodies have not only the faculty of producing heat, but they can also resist heat of a much higher temperature than their own bodies. The heat of the body is kept at its usual standard of 98 degrees, by the exhalent vessels of the skin, which absorb by means of the perspiration the excess of caloric, and fly off with it in a state of vapour.

6. In very hot weather, those functions of the body, such as digestion, nutrition, and secretion, on which the production of animal heat depends, are weakened by the excessive heat, so that in fact less caloric is produced in the system than in cold weather. This is a wise provision of Providence, that when much internal heat is wanted to guard against the cold, it is produced; when less is needed, on account of the warmth, the system generates less.

7. Animal bodies are also capable of developing electricity ; as most of the bodies which surround us are conductors of the electric fluid, it generally passes off as fast as it is formed. Sparks of fire, however, are often seen in winter, on taking off in the dark, silk dresses or stockings, or flannel drawers. Some physiologists maintain that the nervous power is nothing but the electric fluid; but though this may not be admitted, there is a very close analogy between them.

8. It has also been found that needles plunged into the middle of a nerve, become magnetic, and are capable of attracting light substances, such as pieces of paper; and a physiologist by the name of Weinhold says, that he has seen sparks obtained by bringing the divided ends of a nerve together. Muller mentions in his late work on Physiology, that efficient galvanic piles can be constructed from organic substances, without any concurrence of metals. Concentrated solutions of organic substances were spread upon thin paper, and with disks of this, paper piles constructed, the two layers of different substances being separated by two thicknesses of paper; electricity was developed by these piles, and tested by an electrometer. It would seem from these experiments,

that the nervous system is capable of developing electricity, under the influence of vitality.

9. Certain fishes, such as the torpedo, the electrical eel, and five or six other species, are provided with special organs

(In

for the production of electricity. In the torpedo they consist of a large number of prisms, of from three to six sides, standing close together, near the gills of the fish, and perpendicular to the surface. They are composed of membranous tubes, divided into numerous transverse cells, abundantly supplied with blood vessels and nerves. These vessels are filled with a fluid of an albuminous nature. These organs are connected with the brain by three large nerves on each side.)

10. In the gymnotus the electrical apparatus may be compared to a battery of galvanic troughs. Two of these are found on each side of the spine, separated from each other by a long ligament, and extending the whole length of the fish. They are composed of horizontal membranous plates, separated from one another by a small interval, and crossed in a perpendicular direction by membranous partitions, so as to form a great number of cells, which are filled with a gelatinous fluid. These organs receive numerous branches and nerves from the spinal marrow.

11. The resemblance of these organs to the galvanic pile is very striking. The latter consists of alternate plates of copper and zinc with a fluid between them. Thus we have in these fishes alternate layers of membranous partitions and albuminous fluid. But is is worthy of particular notice that these organs lose their electric powers if the nerves which supply them are divided. There can be no doubt then, that the electric shock which they give is a vital act, depending on a nervous influence, and under the control of the animal's will.

12. The sensation produced by the shock of an electrical fish, is also similar to that of common electricity. It is powerful enough to kill small fishes, and is the animal's weapon

of defence. Sparks also have been seen to attend the discharges; and the shock has been communicated through a chain consisting of several persons with their hands joined.

13. Common electricity, and that produced by the organs of electric animals, are generally believed to be different, but it has lately been discovered by Dr. Davy, that the electric organs of the torpedo affect the galvanometer, render needles magnetic, and decompose water

14. Many animals as well as plants are endowed with phosphorescent properties. Every person almost, has seen rotten wood, or dead fish, shine in the dark; such are phosphorescent. (This phenomenon is probably owing to the fact, that during the decomposition of animal or vegetable matter there is formed a highly inflammable compound of carbon, hydrogen, and oxygen, which like phosphorous, burns at the ordinary temperature of the air, with an evolution of light

15. But several living animals exhibit luminous appearances. Many of the lower order of animals that inhabit the sea, and some fishes, are phosphorescent. It is this that causes the luminous appearance of the ocean in warm climates. If a vessel be filled with sea-water containing these animalcula they immediately become phosphorescent, on shaking the vessel.

16. It is not uncommon to find insects which have the faculty of phosphorescence in a high degree. It is supposed to reside in a peculiar albuminous principle, secreted by the animal, and that it requires for its manifestation atmospheric air and a certain degree of heat. It is evidently under the control of the animal's will, and probably depends on the nervous action. Some say, however, that it is a peculiar animal matter secreted at will, which, combining with the oxygen of the air gives rise to the disengagement of light.

Questions.-Have bodies the faculty of regulating their own temper.

ature?70 What is said of animal heat? What is the temperature of cold blooded animals? What preserves the heat of the body uniform? Is as much heat produced in the body in hot as in cold weather? Do animal bodies develope electricity. Can galvanic piles be made of animal substances alone? What kind of fish are electrical? Describe the electrical apparatus of the torpedo. What is a galvanic pile? What effects have the electrical discharges of the torpedo, &c.? In what respects like common electricity? What is said of the phosphorescent properties of animals? What animals have this property? What does it depend on ?

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CHAPTER IX.

THE NERVOUS SYSTEM.

The Brain, Spinal Marrow, and Nerves.

1. THERE is no subject, which falls within the range of physiological enquiry, more interesting or more important, than that of the nervous system. It is this which furnishes the material instrument of thought, the means by which we become acquainted with the external world around us, and also the channels by which the will sends forth its commands to the instruments it employs in their execution.) Besides this, it presides over and regulates the functions of the animal economy, and binds the various organs into one sympathizing, harmonious whole.

2. The nervous system may be considered as embracing, 1st. the brain; 2d. the medulla oblongata; 3d. the spinal marrow; 4th. the nerves; 5th. the ganglia, These are all embraced under two general departments, the first consisting of the brain and its dependencies; and the second, of the ganglia and their nervous connections the first appertaining particularly to animal life, conveying impressions calculated to produce sensations, and sending out volitions to its servants, the organs destined to receive them; while the second supply those organs not under the influence of the will, viz. of digestion, circulation, respiration, and the secretions.

3. These two grand divisions of the nervous system, may be still further distinguished by the circumstance, that the first, or cerebral department, is exactly symmetrical, The nerves all go out in pairs from each side of the brain and spinal cord, while the ganglionic system of nerves is distributed irregularly to all the internal organs. The nerves