TUBULES. 33 



matter; as the states into which this passes in becoming a solid 

 tissue living or capable of life. By the further development 

 of tissue thus far organized, higher or secondary forms are pro- 

 duced, which it will be sufficient in this place merely to enu- 

 merate. Such are, 



4. Filaments, or Fibrils. Threads of exceeding fineness, from 

 770-J Q0th 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 mus- 

 cular 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 es- 

 sential general character distinguished from them.' The flat- 

 tened 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 com- 

 plex; and more compound, so as hardly to permit of being 

 classed as elementary forms, are the striated muscular fibres, 

 which consist of bundles of filaments inclosed in separate mem- 

 branous sheaths, and the cerebro-spinal nerve-fibres, in which 

 similar sheaths inclose apparently two varieties of nerve-sub- 

 stance. 



6. Tubules are formed of simple or structureless membrane, 

 such as the investing sheaths of striated muscular and cerebro- 

 spinal nerve-fibres, and the basement-membrane or proper wall 

 of the fine ducts of secreting glands; or they may be formed, 

 as in the case of the minute capillary lymph and bloodvessels, 

 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 body 

 are built up; the more elementary tissues being, so to speak, 

 first compounded of them ; while these again are variously 

 mixed and interwoven to form more intricate combinations. 

 Thus are constructed epithelium and its modifications, con- 

 nective tissue, fat, cartilage, bone, the fibres of muscle and 

 nerve, &c. ; and these again, with the more simple structures 

 before mentioned, are used as materials wherewith to form ar- 

 teries, veins, and lymphatics, secreting and vascular glands, 

 lungs, heart, liver, and other parts of the body. 



