18 NATURE AND OBJECTS OP PHYSIOLOGICAL SCIENCE. 



1. General Characters of Organized Structures. 



2. Organized structures are characterized, in the first place, by the 

 peculiarities of their form. Wherever a definite form is exhibited by 

 Mineral substances, it is bounded by straight lines and angles, and is 

 the effect of the process termed crystallization. This process results 

 from the tendency which evidently exists in particles of matter, espe- 

 cially when passing gradually from the fluid to the solid state, to arrange 

 themselves in a regular and conformable manner in regard to one ano- 

 ther. There is, perhaps, no inorganic element or combination, which is 

 not capable of assuming such a form, if placed in circumstances adapted 

 to the manifestation of this tendency among its particles ; but if these 

 conditions should be wanting, and the simple cohesive attraction is ex- 

 ercised in bringing them together, without any general control over 

 their direction, an indefinite or shapeless figure is the result. Neither 

 of these conditions finds a parallel in the Organized creation. From 

 the highest to the lowest, we find the shape presenting a determinate 

 character for each species or race, with a certain limited amount of 

 variation amongst individuals ; and this shape is such, that, instead of 

 being circumscribed within plane surfaces, straight lines, and angles, 

 organized bodies are bounded by convex surfaces, and present rounded 

 outlines. We may usually gather, moreover, from their external form, 

 that they are composed of a number of dissimilar parts, or organs; 

 which are combined together in the one individual body, and are cha- 

 racteristic of it. Thus in the Vertebrated or Articulated animal, we at 

 once distinguish the head and extremities from the trunk, which consti- 

 tutes the principal mass ; and where there exist no external organs of 

 such distinctness, as in some Molluscs, the rounded character of the 

 general form is sufficiently characteristic. The very simplest grades of 

 animal and vegetable life present themselves under a shape, which ap- 

 proaches more or less closely to the globular. It is among the lower 

 tribes of both kingdoms, that we find the greatest tendency to irregular 

 departures from the typical form of the species ; and thus is presented 

 an approach, on the one hand, to that inclefiniteness which is characte- 

 ristic of uncrystalline mineral masses ; and, on the other, to that variety 

 of crystalline forms which the same mineral body may present, accord- 

 ing to the circumstances which influence its crystallization. 



3. With regard to size, again, nearly the same remarks apply. The 

 magnitude of Inorganic masses is entirely indeterminate, being altoge- 

 ther dependent upon the number of particles which can be brought 

 together to constitute them. On the other hand, the size of Organized 

 structures is restrained, like their form, within tolerably definite limits, 

 which may nevertheless vary to a certain extent among the individuals 

 of the same species. These limits are least obvious in vegetables, and 

 in the lower classes of animals. A forest-tree may go on extending 

 itself to an almost indefinite extent ; certain species of sea-weed attain 

 a length of many hundred feet, and their growth does not appear to be 

 restrained by any limit ; and the same may be said of those enormous 

 masses of coral, which compose so many islands and reefs in the Poly- 



