*The Cranial Nerves.*-From the under front surface of the brain, twelve pairs of nerves emerge and pa.s.s to the head, neck, and upper portions of the trunk. These, the cranial nerves, have names suggestive of their function or distribution and, in addition, are given numbers which indicate the order in which they leave the brain (Fig. 136). Unlike the spinal nerves, the cranial nerves present great variety among themselves, scarcely any two of them being alike in function or in their connection with different parts of the body. Several of them have to do with the special senses, and are for this reason very important. They connect the brain with the different parts of the head, neck, and trunk, as follows:

1. The first pair (_olfactory_ nerves; nerves of smell; afferent) connect with the mucous membrane of the nostrils (Fig. 136).

2. The second pair (_optic_ nerves; nerves of sight; afferent) connect with the retina of the eyes.

3. The third, fourth, and sixth pairs (_motores oculi;_ control muscles of the eyes; efferent) connect with the internal and external muscles of the eyeb.a.l.l.s (Fig. 136).

[Fig. 136]

Fig. 136-*Diagram suggesting the distribution and functions of the cranial nerves* (Colton). See also Fig. 132.

4. The fifth pair (_trigeminal_ nerves; nerves of feeling to the face, of taste to the front of the tongue, and of control of muscles of mastication; afferent and efferent) connect with the skin of the face, the mucous membrane of the mouth, the teeth, and the muscles of mastication.

5. The seventh pair (_facial_ nerves; control muscles that give the facial expressions; efferent) connect with the muscles just beneath the skin of the face.

6. The eighth pair (_auditory_ nerves; nerves of hearing; afferent) connect with the internal ear.

7. The ninth pair (_glossopharyngeal_ nerves; nerves of taste to back of tongue and of muscular control of pharynx; afferent and efferent) connect with the back surface of the tongue and with the muscles of the pharynx.

8. The tenth pair (_vagus_, or pneumogastric, nerves; nerves of feeling and of muscular control; afferent and efferent) connect with the heart, larynx, lungs, and stomach. They have the widest distribution of any of the cranial nerves.

9. The eleventh pair (_spinal accessory_ nerves; control muscles of neck; efferent) connect with the muscles of the neck.

10. The twelfth pair (_hypoglossal_ nerves; control muscles of the tongue; efferent) connect with the muscles of the tongue.

*Sympathetic Ganglia and Nerves.*-The sympathetic ganglia are found in different parts of the body, and vary in size from those which are half an inch in diameter to those that are smaller than the heads of pins. The largest and most important ones are found in two chains which lie in front, and a little to either side, of the spinal column, and extend from the neck to the region of the pelvis (Figs. 125 and 133). The number of ganglia in each of these chains is about twenty-four. They are connected on either side by the right and left sympathetic nerves which extend vertically from ganglion to ganglion. In addition to the ganglia forming these chains, important ones are found in the head (outside of the cranial cavity) and in the plexuses of the thorax and the abdomen.

The sympathetic ganglia receive nerves from the central division of the nervous system, but connect with glands, blood vessels, and the intestinal walls through fibers from their own cell-bodies. Some of these latter fibers join the spinal nerves, and some blend with each other to form small sympathetic nerves.

*Protection of Brain and Spinal Cord.*-On account of their delicate structure, the brain and spinal cord require the most complete protection.

In the first place, they are surrounded by the bones of the head and spinal column; these not only shield them from the direct effects of physical force, but by their peculiar construction prevent, to a large degree, the pa.s.sage of jars and shocks to the parts within. In the second place, they are surrounded by three separate membranes, as follows:

1. The _dura_, or dura mater, a thick, dense, and tough membrane which lines the bony cavities and forms supporting part.i.tions.

2. The _pia_, or pia mater, a thin, delicate membrane, containing numerous blood vessels, that covers the surface of the brain and cord.

3. The _arachnoid_, a membrane of loose texture, that lies between the dura and the pin.

Finally, within the s.p.a.ces of the arachnoid is a lymph-like liquid which completely envelops the brain and the cord, and which, by serving as a watery cushion, protects them from jars and shocks. Thus the brain and cord are directly shielded by bones, by membranes, and by the liquid which surrounds them. They are also protected from jars resulting from the movements of the body by the general elasticity of the skeleton.

*Summary.*-The nervous system establishes connections between all parts of the body, and provides a stimulus by means of which they are controlled.

It is made up of a special form of cells, called neurons. The neurons form the different divisions of the nervous system, and also serve as the active agents in carrying on its work. Through a side-by-side method of joining they form the nerves, ganglia, spinal cord, and brain; and by a method of end-to-end joining they connect places remote from each other, and provide for nervous movements through the body. The nervous system, may in some respects be compared to a complicated system of telephony, in which the chains of neurons correspond to the wires, and the brain and spinal cord to the central station.

Exercises.-1. Give the meaning of the term "coordination." Supply ill.u.s.trations.

2. What two general conditions are supplied in the body by the nervous system?

3. Compare the skeleton outline of the nervous system with the bony skeleton.

4. Sketch outlines of mon-axonic and di-axonic neurons.

5. Give two differences between the neurons and the other cells of the body.

6. Describe the two general methods of connecting neurons in the body.

What purpose is accomplished by each method?

7. Name and locate the princ.i.p.al divisions of the nervous system.

8. Draw an outline of the brain (side view), locating each of its princ.i.p.al divisions.

9. If a pencil were placed over the ear, what portions of the brain would be above it and what below?

10. Describe briefly the cerebrum, the cerebellum, the midbrain, the pons, and the bulb.

11. Locate and describe the cortex. State purpose of the convolutions.

12. State the general differences between the cranial and the spinal nerves.

13. Locate and give the number of the dorsal-root ganglia. Locate and give the approximate number of the sympathetic ganglia.

14. Show how the two portions of the spinal nerves are formed-the one from the mon-axonic and the other from the di-axonic neurons.

15. Enumerate the different agencies through which the brain and spinal cord are protected.

16. What cranial nerves contain afferent fibers? What ones contain efferent fibers? What ones contain both afferent and efferent fibers?

17. In what respects is the nervous system similar to a system of telephony? In what respects is it different?

PRACTICAL WORK

Examine a model of the brain, identifying the different divisions and noting the position and relative size of the different parts (Fig. 137).

Observe the convolutions of the cerebrum and compare these with the parallel ridges of the cerebellum. If the model is dissectible, study the arrangement of the cell-bodies (gray matter) and the distribution of the fiber bundles (white matter). Note the connection of the cranial nerves with the under side.

[Fig. 137]

Fig. 137-Model for demonstrating the brain (dissectible).

A prepared nervous system of a frog (such as may be obtained from supply houses) should also be examined. Observe the appearance and general distribution of the nerves and their connection with the brain and spinal cord. If such a preparation is not at hand, some small animal may be dissected to show the main divisions of the nervous system, as follows:

*Dissection of the Nervous System* (by the teacher).-For this purpose a half-grown cat is generally the best available material. This should be killed with chloroform and secured to a board as in the dissection of the abdomen (page 169). Open the abdominal cavity and remove the contents, tying the alimentary ca.n.a.l where it is cut, and washing out any blood which may escape. Dissect for the nervous system in the following order: