“If a protein compound be brought into contact with an alkali, ammonia is immediately disengaged, and the alkaline solution can hardly be made weak enough to prevent the disengagement of ammonia. If either fibrin or coagulated albumen be dissolved in a weak potash lye, ammonia is always perceptible. Protein, therefore, is always in a state of decomposition, as serum is alkaline.”
In diagram No. 3, fig. 3, the organic or _involuntary_ muscular fibers of the intestine are shown, consisting of more or less flattened bands, the fibers of which are soft, and marked with minute granules, sometimes exhibiting traces of nuclei. These purely muscular fibers are most abundant next to the inner coat of the artery, and diminish in number as they approach the outer layer, their place being occupied by firmer and more elastic fibers of a yellow color, seen collectively in the circular diagram, as line 4, and separately in diagram 3, fig. 4, and in diagram 4.
The _involuntary_ muscular fibers of an artery do not always form a continuous layer; they are often smaller than those found in the intestines, bladder, and uterus, and occur as fusiform cells, detached from each other, and having a large, club-shaped nucleus, as shown at fig. 6 in diagram 3.
The _voluntary_ muscular fibers differ from the _involuntary_, in having cylindrical fibers of much larger size, with transverse and longitudinal markings, unlike the flattened fibers of less size of the involuntary muscles, which have also a faintly granular appearance, instead of the more determined transverse and longitudinal lines of the voluntary muscles.
The _outer_ or _elastic_ layer of the ancient _middle_ coat, represented by line 4 in the circular diagram, contains muscular fibers, but it is formed principally of strong, elastic fibers difficult of separation, and, when torn across, have curled extremities, as shown in the diagram marked 4, differing only in size from those found in the ligaments of the spine, and in the ligamentum nuchæ of quadrupeds, as shown in the separate diagram marked 4.
The _external_ coat of an artery, divided also into _two_ layers, is shown on the circular diagram by lines 5 and 6. These two layers are composed of the yellow elastic fibers last noticed, and another set of fibers, _white_ in color and _in_elastic in structure, arranged in various directions; the _inner_ layer predominating in yellow elastic, the outer layer in white inelastic fibers, constituting a firm investment to all the other layers of which the artery is composed. The white inelastic fibers are shown in diagram No. 3, fig. 5, with a yellow elastic fiber curling round them. The constant crossing and recrossing of these two sets of fibers form certain spaces, which, when not in a compact form, become real spaces, meshes, or areolæ, constituting what is now called areolar tissue, rather than the cellular of the older anatomists, from the circumstance that the areolæ communicate, and that perfect cells in any tissue do not. These elements of areolar tissue can be readily distinguished by the action of acetic acid, under which reagent the white fibers will almost disappear, leaving only a slight trace of fibers containing oval nuclei, as seen and marked in diagram 3, fig. 5. It is seen when unraveled in _b_, diagram 5.