The middle coat, as it was termed, forms by much the greatest part of the thickness of an artery, and, generally speaking, is of a more or less yellow color. It appears fibrous to the naked eye, and can be peeled off not unfrequently in a series of circular layers; when examined microscopically, it is seen to be composed of _two_ sets of fibers arranged in a circular direction. The inner layer is composed principally of muscular fibers, of the organic or involuntary kind. (See line marked 3 on the circular diagram.) The outer layer, marked line 4 on the same diagram, is made up chiefly of elastic fibers, with a much smaller amount of the muscular or contractile element. These conjoined layers form the muscular coat of Mr. Hunter, the fibrous or contractile coat of later anatomists, who denied its muscularity from the supposed absence of fibrin--an error fallen into from chemical science being unequal at that time to its discovery, or rather of its more elementary part, called _protein_, the principal constituent both of albumen and fibrin, which two are now found to differ from each other in the addition only of three per cent, of sulphur. Mülder says, in his “Animal and Vegetable Chemistry,” (Part II. p. 307:) “The combinations of sulpho-phospho protein (_fibrin_ and _albumen_) and of sulpho-protein _casein_ with acids, alkalies, and salts are especially remarkable. Protein is soluble in weak alkalies. Since, therefore, the serum of the blood is always slightly alkaline, being a proteate of soda, with sulphur and phosphorus, it keeps the sulpho-phospho protein in solution. This property is the cause of the blood remaining in a liquid state--a chief requisite for animal life.
“If a weak alkaline solution of protein be neutralized by an acid, the solubility of sulpho-phospho protein is greatly diminished. The sulphuric and phosphoric acids, by not dissolving protein, stanch bleeding. Acetic acid, by which protein is dissolved, does not, neither does the hydrochloric.
“Protein, according to Mülder--although it is doubted by Liebig--is a complex substance, consisting of several heterogeneous organic compounds united into one whole, easily acted upon by strong reagents.