128 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



structure can be discovered ; and these seem, like the simple fibres 

 already described, to have been formed, rather directly from the nutri- 

 tive fluid, than indirectly by any previous process of transformation. 

 Hence we may regard such membranes and fibres, as constituting the 

 most simple or elementary forms of Animal tissue. The characters of 

 membranes of this kind were first pointed out by Mr. Bowman and 

 Prof. Goodsir ; by the former of whom it was termed basement-mem- 

 brane, as being the foundation or resting-place for the epithelium-cells 

 which cover its free surface ( 231) ; whilst by the latter it was termed 

 the primary membrane, as furnishing the germs of those cells. These 

 terms appear equally appropriate, and may be used indifferently. In 

 its very simplest form, the basement-membrane is a pellicle of such 

 extreme delicacy, that its thickness scarcely admits of being measured ; 

 it is to all appearance perfectly homogeneous, and presents not the 

 slightest trace of structure under the highest powers of the microscope, 

 appearing like a thin film of coagulated gelatine. Examples of this 

 kind may be easily procured, by acting upon the inner layer of any 

 bivalve shell with dilute acid ; this dissolves away the calcareous matter, 

 and leaves the basement-membrane. In other cases, 

 however, the membrane is not so homogeneous ; a 

 number of minute granules being scattered, with 

 more or less of uniformity, through the transparent 

 substance. And we not unfrequently find, in place 

 f these uniformly distributed granules, a series of 

 distinct spots, arranged at equal or variable distances, 

 and in different directions, as shown in Figure 14. 

 Moreover, the membrane thus constituted is disposed 

 to break up into portions of equal size, each of which ' 

 contains one of these spots ; whilst in the more homo- 

 membraneof inhuman geneous forms previously described, we find no such 

 ifcs^wfth^ils' gSnS tendency, no appearance of any definite arrangement 

 spots, or nutritive centres being perceptible when they are torn. Hence it would 



diffused over it. < 1 CL i i ^ c- 



seem as if the first and simplest form were produced 

 by the simple consolidation of a thin layer of homogeneous fluid ; the 

 second, by a layer of such fluid, including nuclear granules ; and the 

 third, by the coalescence of flattened cells, w T hose further development 

 had been checked, but whose nuclei continue to perform their peculiar 

 functions ( 212). We find the primary membrane, under one or other 

 of these forms, on all the free surfaces of the body, beneath the epithe- 

 lial or epidermic cells. Thus, as already mentioned, it constitutes the 

 outer layer of the true Skin ; it lines all the cavities formed by Mucous 

 membranes, and is prolonged into all the ducts and ultimate follicles 

 and tubuli of the Glands which are connected with them ( 199) : indeed 

 it may be said in many instances to be the sole constituent of the walls 

 of these follicles and tubuli, the subjacent tissue not being continued to 

 their finest ramifications. Again, it forms the innermost layer of the 

 Serous and Synovial membranes ; and it also lines the blood-vessels and 

 lymphatics, forming the sole constituent of the walls of their minutest 

 divisions. 



207. In every one of these cases, we find the free aspect of the Pri- 



