3-34. THE SPINAL CORD
a. The spinal cord occupies the upper two-thirds of the spinal canal. It is
continuous with the medulla oblongata and extends from the foramen magnum at the
base of the skull to the cranial border of the second lumbar vertebra. The caudal end
tapers into the filum terminale, which is the threadlike extension of the cord ending at
about the level of the first sacral segment. Thirty-one pairs of spinal nerves extend from
the cord. These nerves exit the neural canal through the intervertebral foramina.
Neurologists have determined approximately what areas of the body are innervated for
feeling (sensory) and motor (muscle contraction), and the level in the spinal cord where
these nerves synapse.
b. The patellar reflex exemplifies the reflex arc. This stretch reflex is obtained
by striking just below the patella with a rubber hammer. The sensory nerve transmits
the impulse through the posterior root of L-2, L-3, and L-4 (which lie in the lower part of
the cord) to connector neurons that terminate in the anterior root. Here, the motor
neurons transmit the impulse to the muscles of the thigh, which contract to "jerk" the
knee. It should be noted that pressure on the cord might result in paralysis or pain. If
the pressure is removed, functions will probably be restored. However, tearing or
breaking the brain or cord results in permanent disability and there is no recovery.
Section V. THE CIRCULATORY SYSTEM
The circulatory system includes the lymphatic system, the blood, and the
cardiovascular system. Its purpose is to carry oxygen and nutrients to the tissues of the
body and to carry carbon dioxide and other waste materials away from the tissue, as
well as to maintain fluid balance and to fight infection.
3-36. THE LYMPHATIC SYSTEM
a. The lymphatic system is composed of the lymphatic vessels and the lymph
glands, or nodes. The lymphatic vessels are similar to veins (they contain valves) and
are filled with lymph. Lymph is interstitial fluid that flows into the lymphatics; interstitial
fluid is the "middle-man" between the blood and the tissues. The microscopic lymph
vessels terminate in the tissues as blind closed-ended tubes. The pressure from the
arterioles forces nutrients into the tissue spaces. This force, plus contracting muscles
and osmosis, pushes some of the fluids into the lymph vessels. The smaller vessels
unite to form larger vessels that carry the lymph into an extensive network of nodes.