28 STRUCTURAL COMPOSITION OF HUMAN BODY. 



becoming a solid tissue living or capable of life. By the 

 further development of tissue thus far organized, higher or 

 secondary forms are produced, which it will be sufficient in 

 this place merely to enumerate. Such are, 



4. Filaments, or fibrils. Threads of exceeding fineness, 

 from ^-.j-jj-jj-th of an inch upwards. Such filaments are 

 cylindriform, as are those of the striated muscular and 

 the fibro-cellular or areolar tissue (fig. 8) ; or flattened, as 

 are those of the organic muscles. Filaments usually lie 

 in parallel fasciculi, as in muscular and tendinous tissues ; 

 but in some instances are matted or reticular with branches 

 and intercommunication, as are the filaments of the middle 

 coat, and of the longitudinally-fibrous coat of arteries ; and 

 in other instances, are spirally wound, or very tortuous, as 

 in the common fibro-cellular-tissue (fig. 9) . 



5 . Fibres in the instances to which the name is commonly 

 applied are larger than filaments or fibrils, but are by no 

 essential general character distinguished from them. The 

 flattened band-like fibres of the coarser varieties of organic 

 muscle or elastic tissue (fig. 10) are the simplest examples 

 of this form ; the toothed fibres of the crystalline lens are 

 more complex ; and more compound, so as hardly to permit 

 of being classed as elementary forms, are the striated mus- 

 cular fibres, which consist of bundles of filaments enclosed 

 in separate membranous sheaths, and the cerebro-spinal 

 nerve-fibres, in which similar sheaths enclose apparently 

 two varieties of nerve substance. 



6. Tubules are formed of simple, or structureless mem- 

 brane, such as the investing sheaths of striated muscular 

 and cerebro-spinal nerve-fibres, and the basement mem- 

 brane or proper wall of the fine ducts of secreting glands ; 

 or they may be formed, as in the case of the minute capil- 

 lary lymph and blood-vessels, by the apposition, edge to 

 edge, in a single layer, of variously shaped flattened cells 

 (fig. 48). 



With these simple materials, the various parts of the 



