418 OF NUTRITION. 



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 Fibrin 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 condition, ever 

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

 that the plasticity of any fluid deposit that is, its capability of being meta- 

 morphosed into organized tissue is in direct relation with the quantity 'of 

 Fibrin which it contains. Thus the Liquor Sanguinis or Coagulable Lymph, 

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

 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 net- 

 work, which fills up the breach, and holds together the surrounding structure. 

 This may be regarded as a simple form of areolar tissue ; which gradually 

 becomes more perfectly organized, by the extension of vessels and nerves into 

 its substance; and in \vhich other forms of tissue may subsequently make 

 their appearance. This process will be more particularly described here- 

 after; it is at present noticed here as an illustration of the general fact, that 

 fibrin is to be regarded as the plastic element of the nutritive fluids. The 

 change from Albumen to Fibrin is, therefore, the first important step in the 

 process of Assimilation. It commences in the Absorbent system, ( 564), 

 and it continues in the Blood ; for the quantity of Fibrin it contains is always 

 kept up, in health, to a certain standard, although there must be a continual 

 withdrawal of it for the nutritive processes, without a correspondingly regular 

 supply from the chyle ; and we find it, moreover, undergoing a sudden and 

 remarkable increase, under the influence of local agencies. The mode in 

 which this conversion is effected, will be better discussed hereafter ; when 

 the chief circumstances under which it occurs, have been inquired into 

 ( 579). 



II. Formation of Cells. 



556. A very large proportion of the Vegetable Organism (in the simplest 

 Plants, the entire structure) is made up of cells or vesicles; wnich are minute 

 closed sacs, whose walls are composed in the first instance of a delicate mem- 

 brane, frequently strengthened, at a period long subsequent to their first 

 formation, by some internal deposit. The form of these cells is extremely 

 variable, and depends chiefly upon the degree and direction of the pressure, 

 to which they have been subjected at the period of their origin, and subse- 

 quently 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 ; these appear to have been 

 formed by the breaking down of the transverse partitions, between a regular 

 series of cylindrical cells laid end to end ; and the remains of such partitions 

 may frequently be seen in them. The ducts which convey the ascending 

 sap do not inosculate with each other, their purpose being merely to carry it 

 direct to the leaves ; but the vessels through which the descending or elabo- 

 rated sap flows, are of very different character; for their purpose is to distribute 

 the nutritious fluid through the tissues ( 497) ; and they anastomose very 

 freely, just as do the capillaries of Animals. The network which they form, 

 however, can be as clearly traced to an origin in cells whose cavities were 

 originally distinct, as can the bundles of straight non-communicating ducts. 

 Another important transformation of the original cells, is that by which the 



