18 ON THE MINUTE STRUCTURE OF 
layer of muscular fibre-cells, each wrapped in a spiral manner round the internal 
membrane, and of sufficient length to encircle it from about one and a half 
to two and a half times. Fig. 18 (Plate II) represents one of these vessels 
as seen under a rather low power, and shows the general spiral arrangement 
of the fibres of the middle coat. Fig. 19 is a camera-lucida sketch of the same 
artery highly magnified, in which I have for the most part traced the outline 
of the fibres on the nearer side of the vessel only, but one fibre-cell is shown 
in its entire length wrapped round nearly two and a half times in a loose spiral. 
In some other vessels the muscular elements were arranged in closer spirals, 
as in Figs. 20and 21. They are seen to have more or less pointed extremities, 
and are provided with an oval nucleus at their broadest part, discernible 
distinctly, though somewhat dimly, without the application of acetic acid. 
The tubular form of the vessels enables the observer, by proper adjustment. 
of the focus, to see the fibre-cells in section; they are then observed to be 
substantial bodies, often as thick as they are broad, though the latter dimen- 
sion generally exceeds the former. Here and there a nucleus is so placed in 
the artery as to appear in section with the fibre-cell, as shown in Figs. 20, 22, 
and 23. The section of the nucleus is in such cases invariably found sur- 
rounded by that of the substance of the fibre-cells, though occasionally placed 
eccentrically in it. From the circular form of its section the nucleus appears 
to be cylindrical. These fibre-cells are from 1-200th of an inch to 1-1ooth of an 
inch in length, from 1-2500th of an inch to 1—2000th of an inch in breadth, and 
about 1-2500th of an inch in thickness, measurements on the whole rather greater 
than those given by Kélliker for the human intestine, the chief difference being 
that in the frog’s arteries they are somewhat broader and thicker. 
Now, the middle coat of the small arteries is universally admitted to be 
composed chiefly of involuntary muscular fibre ; but in the vessels just 
described it consists of nothing whatever else than elongated, tapering bodies, 
corresponding in dimensions with Kolliker’s fibre-cells, and each provided 
with a single cylindrical nucleus embedded in its substance. Considering, then, 
that no tearing of the tissue had been practised in the preparation of the objects, 
but that the parts were seen undisturbed in their natural relations, it appeared 
to me that the simple observation above related settled the point at issue con- 
clusively. 
It was, however, suggested to me by an eminent physiologist, that the 
various forms in which contractile tissue occurs in the animal kingdom forbid 
our drawing any positive inference regarding the structure of human involuntary 
muscle from an observation made on the arteries of the frog. Being anxious 
to avoid all cavil, and understanding that Mr. Ellis’s researches had been 
