38 Rev. P. Keith on the Conditions of Life. 



planl, an animal. An organ is a fabric adapted by its struc- 

 ture to the performance of a function; — a hand, a foot, a leaf, 

 — prehension, progression, aeration, Bichat has remarked 

 that the organs destined to the higher functions of the higher 

 orders of animals — the organs by which they communicate 

 with the external world — are more symmetrical in their form 

 than other organs, and many of them double, as the eyes, the 

 ears, the hands, the feet; or divisible into two corresponding 

 halves, with a manifest medial line, — as the brain, the tongue. 

 The higher the function the greater the symmetry of the or- 

 gan *. \A'^e may extend this remark to vegetables also. It will 

 not be found to apply so generally, nor in the same degree ; but 

 it is easy to discern traces of the fact. The leaves and petals 

 are among the most important of all vegetable organs, and they 

 show this peculiarity very conspicuously. They are divisible 

 into an anterior and posterior surface ; and into a right and 

 left side separated by the intervention of a midrib. The in- 

 terior organs of the flower may be regarded as divisible into 

 two equal and similar halves ; the spongiolae of the radicles 

 may be regarded in the same light; and perhaps the beautifully 

 twisted form of the spiral threads should be regarded also 

 as an example of the symmetry in question. An assemblage 

 of several organs all concurring to the production of a single 

 result constitutes an apparatus, — the visual apparatus, the di- 

 gestive apparatus, the lacteal apparatus. An assemblage of 

 organs possessing the same or a similar structure constitutes 

 a system, — the vascular system, the osseous system, the nervous 

 system. The immediate constituents of organs are tissues, — 

 the cellular tissue, or the fibrous tissue. 



Of all living bodies whether plants or animals, the principal 

 mass is composed of the cellular tissue. It enters into the 

 composition of almost every organ, and binds and cements 

 together the fibres that pervade it. Particularly, it forms 

 the principal mass of succulent plants, and a notable portion 

 of many parts of woody plants. It abounds in succulent 

 fruits, and in the lobes of all seeds. It consists of clusters 

 of little cells or vesicles containing an inclosed fluid, which 

 Grew compared in their aggregate aspect to the bubbles formed 

 upoji the surface of liquors in a state of fermentation. — The 

 foetus seems a homogeneous mass of cellular tissue filled with 

 a gelatinous fluid. As the organs begin to show themselves 

 this mass becomes more condensed. As the bulk of the organs 

 increases, the proportional bulk of the tissue diminishes. It 

 »s the receptiicle of lymph and of fat, and is at all periods of 



' Rcchcrc/iCi Phijsiologiqncs, par Xav. Bichat. 8. 



life 



