421. What is said respecting secreted substances? Do vegetables possess the property of secretion? 422. From what are the various textures formed? 423. Have we accurate knowledge of the agents that produce secretion?
424. Secretions are constantly maintained, during life, from the serous membrane, by the action of the internal exhalants. The fluid which is exhaled bears some resemblance to the serum of the blood. Its use is to furnish the organs, which are surrounded by this membrane, with a proper degree of moisture, and thus enables them to move easily on each other, as those within the chest and abdomen.
425. The cellular tissue exhales a serous fluid, and when it becomes excessive in quantity, general dropsy is produced. Fat is another secretion, which is thrown out, in a fluid state, from the cellular membrane. It is deposited in little cells, and exists in the greatest abundance between the skin and the muscles. Its use seems to be, to form a cushion around the body for its protection; to furnish nutriment for the system when food cannot be taken; to supply the carbon and hydrogen necessary to sustain the generation of heat, when these articles of combustion are not otherwise furnished. The med´ul-la-ry substance, (marrow,) in the cavities of the long bones, is very much like fat.
Observation. During sickness, if there is not emaciation or absorption of this secretion, it is considered an unfavorable symptom, because it indicates a want of power in the absorbing system, which is among the last to be affected.
How is it proved that secretion depends on nervous influence? 424. What is said of the secretions from the serous membrane? 425. From what tissue is a serous fluid exhaled? What is the effect when this fluid becomes excessive in quantity? What is fat? Its use? What is marrow?
426. The mucous secretion is a transparent, viscid fluid which is secreted by those membranes that line the cavities of the body, which have an external communication, as the trachea and alimentary canal. This secretion serves to protect these parts from the influence of the air, and concurs, by means of its peculiar properties, in the performance of their functions. 427. There are two external secretions, namely, one from the skin, called perspiration, and the other from the lungs. The cutaneous exhalation, or transpiration[14] exists in two forms, called sensible perspiration (sweat) and insensible perspiration. The pulmonary exhalation is the most important and universal, and closely resembles that of the skin.
428. The follicles are found only in the skin and mucous membrane. They secrete an oily, unctuous substance, which mixes with the transpiration, and lubricates the skin. At the root of each hair there is a minute follicle, which secretes the fluid that oils the hair. The wax in the passage of the ear is secreted from these bodies.
429. All the blood distributed to the different glands is similar in composition and character; but the fluids secreted by them, vary in appearance in a remarkable degree. The office of the glands appears to be principally to form different secretions. Thus the salivary glands secrete the insipid saliva; the lachrymal glands, the saline tears; the liver, the yellow, ropy bile; and the kidneys, the acrid urine.
426. What is said relative to the mucous secretion? 427. Name the external secretions. 428. Give the office of the follicles. 429. What appears to be the principal office of the glands? 430. Mention a secretion produced in a particular emergency.