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. [14] _Transpiration_ is a term often used generically, to signify the passage of fluids or gases through membranes, internally or externally; but _perspiration_ is a specific term, signifying transpiration on to the external surface. 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. -=-=-=-=-=-=-=-=-=-=-=-= 430. Some secretions are evidently produced only in particular emergencies, as is seen in the increased secretion of bony matter when a limb is broken. 431. When any substance which is not demanded for nutrition, or does not give nourishment to the system, is imbibed by the lymphatic vessels, and conveyed into the blood, it is eliminated in the secretions.