ANIMAL SECRETION

ANIMAL SECRETION is the Act whereby the divers Juices of the Body are secreted or separated from the common Mass of Blood, by means of the Glands.
See SECRETION, BLOOD, and GLAND.
The Theory of Animal Secretion is one of the Improvements in Physic, for which we are indebted to Geometrical Reasoning.

The sum of what our late Writers on the subject have shown, may be reduced to the following heads:(1.) Different Juices may be separated from the common Mass of Blood by means of the different Diameters of the Orifices of the secretory Ducts.



For all Particles, whose Diameters are less than those of the Ducts, will pass through them; so that any Matter may be evacuated by any of the Glands, provided the Diameters of its Particles be made lesser than those of the secretory Duct, either by a Comminution of the Matter to be separated, or by an Enlargement of the separating Passage.

(2.) By the different Angle which the secretory Duct makes with the Trunk of the Artery. For all Fluids press the Sides of the containing Vessels in a Direction perpendicular to its Sides; which is evident in the Pulsation of the Arteries, it being to that Pressure that the Pulsation is owing. It is likewise evident, that the Blood is urged forward by the Force of the Heart; so that the Motion of Secretion is compounded of both these Motions. Now the lateral Pressure is greater when the direct Velocity is so; but yet not in proportion to such Velocity: for the lateral Pressure is considerable, even when the fluid is at rest; being then in proportion to the Specific Gravity of the Fluid: And in a Fluid like the Blood in the Arteries which is thrown in a right Direction, or a Direction parallel to the Axis of the Vessel, the lateral Pressure will be in a Proportion compounded of both: From whence it will follow, that if two Particles of equal Diameters, but of unequal Specific Gravities, arrive, with the same Velocity, at an Orifice capable of admitting them, yet they will not both enter it and pass, because their Motion of Direction will be different: So that the Diversity of the Angles which the Ducts make with the Trunk of the Artery, is altogether necessary to account for all the possible Diversities of secreted Fluids, even supposing their Diameters and Figures to be the same.

(3.) By the different Velocities with which the Blood arrives at the Orifices of the secretory Ducts. For since the Secretions are made in a fluid form; no possible Reason can be assigned, why some Animals have a soft loose Texture of the solid Parts, and why one Part of the Body is of an easily separated Texture, and others of a firmer, but this different Velocity of the Blood at the Orifices of the secretory Ducts, whereby the Particles secreted for Nourishment, and Accretion, are driven or impacted into the Vacuola, that receive them with a greater or less Force; it being difficult to imagine that such a Diversity in Texture can altogether proceed from the different Solidities and Contacts of the constituent Parts. See further under the Article SECRETION.

Animal Spirits, are a fine subtle Juice, or Humour in Animal Bodies; supposed to be the great Instrument of muscular Motion, Sensation, etc. See MUSCULAR MOTION, SENSATION, etc. The Ancients distinguished Spirits into three Kinds, viz. Animal, Vital, and Vegetative: but the Moderns have reduced them to one sort, viz. Animal; about the Nature of which, and the Matter whence they are formed, great disputes have arisen among the Anatomists, though their very Existence has never been fairly proved. As it is hard to define what could never yet be brought under the Judgment of our Senses, all that we shall here offer concerning them, is, that they must needs be extremely subtle Bodies, which escape all manner of Examination by the Senses, though ever so well assisted; and pervade the tracts of the Nerves, which yet have no discoverable Cavity or Perforation; nor could ever by any Experiment be collected; yet are constantly moving in vast Quantities, as they must of necessity be, to perform all those mighty Operations which are ascribed to them—However, the Antiquity of the Opinion claims some Reverence. By the Help of these we are furnished with a vast Number of precarious Solutions of great Phenomena; and without them we must leave a great Chasm in the philosophical History of Animal Bodies. They are supposed to be separated in the Brain, from the subtlest Parts of the Blood; and thence carried, by the Nerves, to all Parts of the Body, for the Performance of all animal and vital Functions. See BRAIN and NERVE.

See also further under the Article SPIRIT.

ANIMAL SECRETION