Its action, however in such cases is much weakened. That the heart is, however, in some measure influenced by the spinal innervation, must be admitted.

16. (What is called the capillary circulation, or that in the small vessels, is much under the influence of the spinal mar. row. It is found that when any part of it is destroyed, the blood does not, circulate in the small vessels which derive their nerves from the portion destroyed. In these cases, the skin becomes purple, and dry; perspiration ceases, the cuticle peels off, and the part becomes sensibly colder. The whole nervous system, however, is concerned in the production of animal heat.

17 Digestion is to a considerable degree under the influence of the spinal cord) (In all cases of diseases of the spine, the appetite is poor, and the digestive powers weak. Colic and dyspepsia are frequent attendants upon such complaints. If the eighth pair of nerves be divided, the stomach is immediately paralyzed, and digestion interrupted. This shows that the stomach depends for nervous influence on the me. dulla oblongata, and that it is derived through the medium of the par vagum.

18. The spinal marrow exerts an important influence over the kidneys, more so perhaps than over any of the abdominal viscera. Some physiologists also think that it presides over the functions of nutrition. What seems to confirm this opinion, is the fact, that in the crustacea, insects, and worms, which have the power of speedily reproducing any part that is cut off, the spinal marrow preponderates over every other portion of the nervous system. Nutrition is known to depend chiefly on the influence of the great sympathetic nerve, but its connections with the spinal marrow are so numerous and intimate, that there can be little doubt, it derives a considerable portion of its nervous energy from the latter. The following cut represents the left side of the brain and spinal marrow, shown by making a section of the cranium and the spinal column, and removing the dura mater.

a. The convolutions of the cerebrum.

b. The lamina of the cerebellum.

e. The pons varolii.

g. The medulla oblongata.

c. d. f. The medulla spinalis, or spinal marrow, extending from the first cervical to the first lumbar vertebra, and terminating in the cauda equina. The cerebrum, it will be seen, is the largest portion of the brain, and occupies the whole upper cavity of the skull. It rests anteriorily upon the arches of the orbits; in the centre, upon the middle fossac of the base of the skull, and posteriorily upon the tentorium cerebelli.

Questions.-How do we find the brain developed in the lower animals? Have they great tenacity of life? Will zoophytes live when divided into pieces 7 What is the medulla oblongata? What its func. tion? Describe the spinal cord ?-its membranes ? its shape ?-its structure? How many pair of nerves arise from it? How many from the neck—the back? the loins? What is the function of the ante. rior roots?—the posterior roots? What is the use of the spinal cord? How is this illustrated? How proved? What follows, if the spinal cord be severed opposite the second bone of the neck?-the fifth ?—the first dorsal? Does the principle of motion reside to any extent in the spinal cord itself? What experiment proves this? Why does a divi. sion of the par vagum or eighth pair cause death? Does the spinal mar. row exert any influence on the circulation of the blood?on the capil. lary circulation on digestion? What proves this? Has it any in. fluence over the kidneys ?over nutrition?

CHAPTER XII.

THE NERVES AND THEIR FUNCTIONS.

1. In describing the nerves and their functions, it will be necessary to classify them, or to reduce them to such arrangement, as the present state of our knowledge on the subject will admit. The following comes as near such a classification as it is possible to make :-1 Nerves of Special Sense. 2. Motific Nerves. 3. The Respiratory Nerves. 4. The Regular Nerves, 5. The Ganglionic Nerves

2. FIRST ORDER. The nerves which contribute to the senses of sight, hearing, and smell, constitute the first class, and are called Nerves of Specific Sense; because they are incapable of communicating any other impressions than such as belong to the respective senses with which they are connected. For example, the olfactory can only transmit ideas of odours, the optic of colours, the auditory of vibrations, no matter what the stimulants may be which excite them to action. If we send an electric shock to the eye, it receives the impression of light; to the ear, of sound; and to the nose, an odour is perceived: so also, if we press or strike the ball of the eye, we experience a vivid sensation of light, and the same phenomenon occurs when the eye is pierced by the needle of the surgeon: so also, if we take two pieces of different kinds of metal in the mouth, and placing one over, and the other under the tongue-allow their edges to come together, we perceive at once a peculiar taste, excited by galvanism. Thus do these nerves communicate to the mind that they are affected in the only manner of which they are capable.

This cut represents a portion of nerve, showing the filaments of which it is composed, and one of them drawn out.

3. First, or olfactory.-These nerves take their name from " olfactus," the smell, as they are essential to this sense. They are the softest nerves in the body, and more closely connected with the hemispheres of the brain than any other! They lie immediately under the anterior lobes of the brain; and as they pass on, they swell into a bulb, from which numerous fibres issue, and pass into the nose through a plate of one of the bones of the skull. This plate is pierced with so many holes for transmitting these twigs of the nerve, that it has the appearance of a seive; and from this circumstance is called the ethmoid bone. The olfactory nerve is spread out upon the lining membrane of the nose, and is merely defended by a thin layer of mucus. It was necessary that

these nerves should come to the surface, in order to come in contact with the vapours inhaled by the nostrils. In some animals these nerves are very large, especially in ruminants, but in others they are entirely wanting, as in whales. In fishes, you may see the bulb of the olfactory placed immediately under the cup-like nostril. From the bulb, the nerve runs backward along a canal filled with transparent fluid, enters the skull, and joins the brain.

4. The second, or optic. These are the largest of the cerebral nerves, and can be traced as far back as the medulla oblongata at the base of the brain. They pass along the base of the brain, and just before they enter the orbits they unite with each other) or seem to decussate, or cross each other. In some fishes, these nerves evidently cross without union; but in man, it is now pretty well established, that a semidecussation takes place; that is, one half of the right optic nerve crosses to the opposite side, and joins a half of the