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end plates ;" when the nerve enters the muscular fiber the tubular membrane blends with the sarcolemma, the medullary layer disappears, and the axis cylinder spreads out into the form of a little plate, granular in character, and containing oval nuclei.
In the unstriped or involuntary muscles, the terminal nerve fibers form a plexus on the muscular fiber cells, and become connected with the granular contents of the nuclei.
In the glands nerve fibers have been traced to the glandular cells, where they form a branching plexus from which fibers pass into their interior and become connected with their substance, and thus influence secretion.
Sensitive Nerves terminate in the skin and mucous membranes, in three distinct modes, e.g., as tactile corpuscles, Pacinian corpuscles, and as end bulbs.
The tactile corpuscles are found in the papillæ of the true skin, especially on the palmar surface of the hands and fingers, feet and toes; they are oblong bodies, measuring about go of an inch in length, consisting of a central bulb of homogeneous connective tissue surrounded by elastic fibers and elongated nuclei. The nerve fiber approaches the base of the corpuscle, makes two or three spiral turns around it, and terminates in loops. They are connected with the sense of touch.
The Pacinian corpuscles are found chiefly in the subcutaneous cellular tissue, on the nerves of the hands and feet, the intercostal nerves, the cutaneous nerves, and in many other situations. They are oval in shape, measure about the lo of an inch in length on the average, and consist of concentric layers of connective tissue; the nerve fiber penetrates the corpuscle and terminates in a rounded knob in the central bulb. Their function is unknown.
The end bulbs of Krause are formed of a capsule of connective tissue in which the nerve fiber terminates in a coiled mass or bulbous extremity; they exist in the conjunctiva, tongue, glans penis, clitoris, etc.
Many sensitive nerves terminate in the papillæ at the base of the hair follicle ; but in the skin, mucous membranes, and organs of special sense their mode of termination is not well understood.
PROPERTIES AND FUNCTIONS OF NERVES. Classification.-Nerves may be divided into two groups, viz. :
(1) Afferent or centripetal, as when they convey to the nerve centers the impressions which are made upon their peripheral extremities or parts of their course. They may be sensitive, when they transmit impressions which give rise to sensations; reflective or excitant, when the impression carried to the nerve center is reflected outward by an efferent nerve and produces motion or some other effect in the part to which the nerve is distributed.
(2) Efferent or centrifugal, as when the impulses generated in the centers are transmitted outward to the muscles and various organs. They may be motor, as when they convey impulses to the voluntary and involuntary muscles; vasomotor, when they regulate the caliber of the small blood-vessels, increasing or diminishing the amount of blood to a part ; secretory, when they influence secretion; trophic, when they influence nutrition; inhibitory, when they conduct impulses which produce a restraining or inhibiting action.
Irritability, Excitability, Neurility.--All nerves possess the property of being called into action by a stimulus in virtue of the possession of an ultimate and inherent property denominated irritability or excitability, which is manifested so long as the physical and chemical integrity of the nerve is maintained. During the period of excitement, no change in the nerve is appreciable except an electrical one.
The irritability of a motor nerve is demonstrated by the contraction of the muscles to which it is distributed; the impulse aroused by an irritant travels outward to the muscles and calls forth a contraction.
The irritability of a sensory nerve is demonstrated by the development of a conscious sensation. An irritation applied to a sensory nerve in any part of its course arouses an impulse which travels to the brain and produces there a sensation. The irritability of a sensory nerve may be increased by congestion or inflammation and decreased by cold, compression, and injuries. Other tissues—. g., muscles, glands, etc.—possess irritability, and when subjected to the action of a stimulus react in their own particular way. The irritability of nerves is distinct and independent of the irritability of muscles and glands, as can be demonstrated by the use of poisons, such as woorara, atropia, etc.
The properties of sensation and motion reside in different nerve fibers. Motor nerves can be destroyed or paralyzed by the introduction of woorara under the skin, without affecting sensation; the sensibility of nerves can be abolished by the employment of anesthetics without destroying motion.
In the transmission of the nerve impulse the axis cylinder is the essential conducting agent, the white substance of Schwann and tubular membrane being probably accessory structures, protecting the axis from injury, and preventing the diffusion of nerve force to adjoining nerves.