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 -* T inch to ^ inch in length, from 

 jf^ inch to -^~- inch in breadth, and about ~s inch in thickness, mea- 

 surements on the whole rather greater than those given by Kolliker 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 Kolliker'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 Ellis'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 pupillse, except that the appearances were more distinct, espe- 

 cially as regards the nuclei, which were clearly apparent without the application 

 of acetic acid. Several of the fibre-cells were isolated in the first specimen I ex- 



