552 MR LISTER ON THE MINUTE STRUCTURE OF 
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 dimension 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-cell, though occasionally placed ec- 
centrically in it. From the circular form of its section the nucleus appears to be 
cylindrical. These fibre-cells are from ,}; inch to ;}, inch in length, from 
100 
sa, inch to =, inch in breadth, and about ;,,, inch in thickness, mea- 
surements on the whole rather greater than those given by KotiiKxer for the hu- 
man intestine, the chief difference being that in the frog’s arteries they are some- 
what broader and thicker. 
Now, the middle coat of the small arteries is universally admitted to be com- 
posed 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 KoLiikEr’s fibre-cells, and each provided with a single cylin- 
drical 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 sim- 
ple observation above related settled the point at issue conclusively. 
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 Ettts’s researches had been 
directed chiefly to the hollow viscera, I thought it best to examine the tissue 
in some such organ. For this purpose I obtained a portion of the small intes- 
tine of a freshly killed pig, selecting that animal on account of the close ge- 
neral resemblance between its tissues and those of man. The piece of gut hap- 
pened to be tightly contracted, and on slitting it up longitudinally, the mucous 
membrane, which was thrown into loose folds, was very readily detached from the 
subjacent parts. I raised one of the thick, but pale and soft fasciculi of the cir- 
cular coat, and teased it out with needles in a drop of water, reducing it without 
difficulty to extremely delicate fibrils. On examining the object with the micro- 
scope, I found that it was composed of involuntary muscular fibre, almost entirely 
unmixed with other tissue, reminding me precisely of what I had seen in the hu- 
man sphincter pupillee, except that the appearances were more distinct, espe- 
cially as regards the nuclei, which were clearly apparent without the application ‘i 
of acetic acid. Several of the fibre-cells were isolated in the first specimen I ex- 
