Each nerve is composed of delicate filaments, or fibriUtc, which vary in thickness from a hair to silk fiber. These filaments are enclosed in a cellular sheath called neurilema and collected into fasciculi. The fasciculi can range in size but generally consist of several bundles. Neurilema forms small channels that can be rendered visible by injecting quicksilver after dissolving the nervous matter with diluted alkali. A loose cellular tissue connects these sheaths, sometimes accumulating a small quantity of fat. Proper nervous matter may be obtained directly from the neurilema using nitric or muriatic acid and is thought to consist of the fibrous or white substance of the brain. The arteries of nerves are large and numerous, penetrating between their fibers; each vessel subdivides into two branches, one following the nerve's course while the other moves in a retrograde direction. When injected successfully, nerves appear red due to blood-filled veins. Absorbent ramifications can only be traced in large nerves, with some anatomists denying their existence altogether.
The density of nerves varies depending on the strength of the neurilema, which thickens and strengthens in exposed areas like limbs but is delicate in optic and auditory nerves. Nerves are not elastic; when irritated in a living animal, no movement or vibration can be perceived. The primary function of nerves is to receive and transmit impressions from the brain to other parts of the system. Volitions from the mind travel rapidly from the encephalon to muscles, while external stimuli are conveyed back to the sensory center.
Anatomists have long debated whether there are specific nerves for sensation and motion. It was discovered that some nerves were entirely connected with sensation (first, second, third, fourth, sixth, and seventh pairs), while others were motor nerves (third, fourth, sixth, and twelfth pairs). Mr. Bell's experiments proved the difference in functions between anterior and posterior roots of spinal nerves: touching the posterior caused muscle contractions without convulsions; touching the anterior did not cause immediate contraction.
The power of nerves appears to be restricted to that of conductors, distinct from brain function. Sir Gilbert Blane’s experiments showed that after removing a kitten's head or dividing its spinal cord, hind legs responded immediately to touch with hot wire, proving nerve transmission independent of brain involvement. Mr. Mayo observed that motor and sensory nerves originate in the same regions.
Nervous ganglia are round or oval hollows found along the course of nerves but only in the body trunk; limbs lack them. Ganglia have been divided into simple (on spinal nerves) and compound (on sympathetic nerves). Simple ganglia are constant in form and location, while compound ganglia vary significantly.
<Callout type="important" title="Key Discovery">Mr. Bell's experiments proved that the anterior roots of spinal nerves control motion, while posterior roots handle sensation.</Callout>
The structure of ganglia has been meticulously examined by several anatomists; Monro and Scarpa’s observations are considered accurate and satisfactory. Ganglia consist of two substances: medullary (fibrous) and peculiar pulpy matter surrounding nervous filaments.
Key Takeaways
- Nerves are composed of delicate filaments enclosed in a cellular sheath called neurilema.
- The primary functions of nerves include receiving and transmitting sensory and motor signals.
- Ganglia, found along the course of nerves, vary in structure but play crucial roles in nerve function.
Practical Tips
- Understanding the basic anatomy of nerves can aid in recognizing symptoms of neurological disorders early on.
- Knowledge of ganglion locations is essential for surgical procedures involving nerves.
Warnings & Risks
- Manipulating or damaging a nerve without proper knowledge can lead to severe complications such as paralysis.
- Ignoring the distinct functions of anterior and posterior spinal roots can result in misdiagnosis of neurological conditions.
Modern Application
While this chapter provides foundational anatomical information on nerves, modern medical imaging techniques like MRI and CT scans offer more precise insights into nerve structures. However, understanding historical perspectives enhances appreciation for advancements in neurology.
Frequently Asked Questions
Q: What are the primary functions of nerves according to the text?
The primary functions of nerves include receiving and transmitting sensory and motor signals between different parts of the body and the brain.
Q: Who was credited with discovering the distinct functions of anterior and posterior spinal roots?
Mr. Bell's experiments proved that the anterior roots control motion, while the posterior roots handle sensation in spinal nerves.
Q: What is the significance of ganglia in nerve function?
Ganglia are round or oval hollows found along the course of nerves and play crucial roles in transmitting signals. They vary significantly in structure but are essential for proper nerve function.