108 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



the bird. It is probably in the same manner, that the Chorion of the Mam- 

 miferous animal originates ; since this is a new envelope, formed around the 

 ovum, during its passage along the Fallopian tube. In the latter, for an ulte- 

 rior purpose, vessels are afterwards developed, by extension from the con- 

 tained ovum; and by the nutrition they supply, its size is increased, and 

 changes take place in its texture. But in the Egg-membrane of the Bird, 

 there is no need of vessels ; because no subsequent change in its texture is 

 required, and its duration is sufficient for the purpose it has to answer. 



119. The completeness of the transformation of Fibrine into simple Fi- 

 brous Tissue, appears to depend upon two circumstances in particular ; the 

 perfect elaboration of the Fibrine itself; and the vitality of the surface upon 

 which the concretion takes place. When the Fibrine is highly elaborated, it 

 will coagulate in the form of a definite network of minute fibrillae, even upon 

 a dead surface, as a slip of glass ; this is the case, for instance, with the Fi- 

 brine of the bufTy coat of the Blood, or with that of the Liquor Sanguinis 

 (coagulable lymph) poured out for the reparation of an injured part. But in 

 the ordinary Fibrine of the blood, the fibrillation is less distinct when the con- 

 cretion takes place upon a dead surface. When it occurs in contact with a 

 living surface, however, the coagulation takes place more gradually ; and it 

 seems as if the particles, having more time to arrange themselves, become ag- 

 gregated into more definite forms, so that a more regular tissue is produced 

 just as crystals are most perfectly formed when the crystalline action takes 

 place slowly. It was formerly imagined that the Muscular tissue is the only 

 one produced at the expense of the Fibrine of the blood ; the other tissues 

 being formed from its Albumen. This, however, is unquestionably erroneous. 

 There is no proof whatever that Albumen, as long as it remains in that con- 

 dition, ever becomes organized ; whilst, on the other hand, there is abundant 

 evidence, that the plasticity of any fluid deposit that is, its capability of be- 

 ing metamorphosed into organized tissue is in direct relation with the quan- 

 tity of Fibrine which it contains. Thus the Liquor Sanguinis, or Coagulable 

 Lymph, thrown out for the reparation of injuries, contains a large amount of 

 Fibrine ; and this substance is converted, not at first into muscular fibre, but 

 (whatever may be the tissue to be ultimately produced in its place) into a 

 fibrous network, which fills up the breach and holds together the surrounding 

 structure. This may be regarded as a simple form of areolar tissue ; Avhich 

 gradually becomes more perfectly organized by the extension of vessels and 

 nerves into its substance ; and in which other forms of tissue may subse- 

 quently make their appearance. This process will be more particularly de- 

 scribed hereafter ; it is at present noticed here as an illustration of the general 

 fact, that Jib rine is to be regarded as the plastic element of the nutritive fluids. 



3. Of the Elementary Parts of Organized Tissues; Cells, Membrane, 



and Fibre. 



120. The cells, which have been spoken of as making up the chief part of 

 the Vegetable Organism, are minute closed sacs; whose walls are composed 

 in the first instance of a delicate membrane, frequently strengthened, at a 

 period long subsequent to their first formation, by some internal deposit. The 

 form of these cells is extremly variable ; and depends chiefly upon the degree 

 and direction of the pressure, to which they may have been subjected at the 

 period of their origin, and subsequently to it. Sometimes they are spheroidal ; 

 sometimes cubical or prismatic ; sometimes cylindrical ; and sometimes very 

 much prolonged. These cells may undergo various transformations. One of 

 the most common, is the conversion of several into a continuous tube or Duct. 

 This is principally seen in the vessels, through which the sap ascends the stem ; 



