Structure of the Nervous System : Part 6

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 22 of 28)

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. 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.

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. - 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. Open the abdominal cavity and remove the contents, tying the alimentary canal where it is cut, and washing out any blood which may escape. Dissect for the nervous system in the following order:

1. Cut away the front of the chest, exposing the heart and lungs. Find on each side of the heart a nerve which passes by the side of the pericardium to the diaphragm. These nerves assist in controlling respiration and are called the phrenic nerves. Find other nerves going to different parts of the thorax.

2. Remove the heart and lungs. Find in the back part of the thoracic cavity, on each side of the spinal column, a number of small "knots" of nervous matter joined together by a single nerve. These are sympathetic ganglia. Where the neck joins the thorax, find two sympathetic ganglia much larger than the others.

3. Cut away the skin from the shoulder and upper side of the fore leg. By separating the muscles and connective tissue where the leg joins the thorax, find several nerves of considerable size. These connect with each other, forming a network called the brachial plexus. From here nerves pass to the thorax and to the fore leg.

4. From the brachial plexus trace out the nerves which pass to different parts of the fore leg. In doing this separate the muscles with the fingers and use the knife only where it is necessary to expose the nerves. Note that some of the branches pass into the muscles, while others connect with the skin.

5. Remove the skin from the upper portion of one of the hind legs and separate the muscles carefully until a large nerve is found. This is one of the divisions of the sciatic nerve. Carefully trace it to the spinal cord, cutting away the bone where necessary, and find the connections of its branches with the cord. Then trace it toward the foot, discovering its branches to different muscles and to the skin.

6. Unjoint the neck and remove the head. Examine the spinal cord where exposed. Cut away the bone sufficiently to show the connection between the cord and one of the spinal nerves. On the dorsal root of one of the nerves find a small ganglion. What is it called?

7. Fasten the head to a small board and remove the scalp. Saw through the skull bones in several directions. Pry off the small pieces of bones, exposing the upper surface of the brain. Study its membranes, convolutions, and divisions.

8. With a pair of bone forceps, or nippers, break away the skull until the entire brain can be removed from the cavity. Examine the different divisions, noting the relative position and size of the parts.

9. With a sharp knife cut sections through the different parts, showing the positions of the "gray matter" and of the "white matter."

NOTE. - If the entire class is to examine one specimen, it is generally better to have the dissecting done beforehand and the parts separated and tacked to small boards. This will permit of individual examination. Sketches of the sciatic nerve, brachial plexus, and of sections through the brain and spinal cord should be made.

Location of Nerves in the Body. - Several of the nerves of the body lie sufficiently near the surface to be located by pressure and are easily recognized as sensitive cords. Slight pressure from the fingers reveals the presence of nerves in the grooves of the elbow, between the muscles on the inner side of the arm near the shoulder, and in the hollow part of the leg back of the knee. These are all large nerves. Small nerves may be located in the same manner in the face and neck.

D.C. Heath and Co. - Publishers
Original copyright 1909