382 
MR. SKEY ON MUSCULAR FIBRE. 
which has presented material of the deepest interest to all physiologists of the last 
and the present century. 
If a portion of the middle coat of an artery, whether of the pulmonary or aortic 
system, be submitted for examination, it is impossible to distinguish it from the mus- 
cular texture of the stomach, intestinal canal, or bladder. It exhibits the perfect 
composition of the organic muscular texture of these parts (Plate XIX. fig. 2.). 
It would be impracticable to determine, with so large a microscopic power as that 
which this subject demands, the relative proportions of muscular fibre in the larger, 
compared with that in the smaller arterial tubes ; but I have observed that the mus- 
cular texture of the smaller vessels, as the internal mammary and the smaller branches 
of the iliacs, is paler and of a more delicate fabric, but their relative proportions could 
only be appreciated by a different mode of inquiry. 
I need hardly state, perhaps, that the fibres are placed circularly around the ves- 
sels, and that the muscular, forms the thickest of the coats of these tubes. 
I can discover no resemblance between the structure of the middle coat of an artery 
and that of the elastic ligamentous tissues of the body. 
If the drawing of the former be compared with that of the muscular fibre of or- 
ganic life in general, I think it will be found so closely to correspond as to appear 
almost identical. Possibly the arterial tissue is more delicate, but both apparent 
composition and arrangement are the same. 
I observe, however, no comparison between the arterial tissue and that of the elastic 
ligamentous structures. These latter are composed of large and distinct filaments 
placed in a parallel direction, and connected by dense cellular tissue. Each filament 
possesses its characteristic property of elasticity, and when separated at one extremity 
from the mass it curls backwards on itself. 
The entire structure is likewise more transparent than the arterial tissue, and is 
much more simple in its arrangement (Plate XIX. fig. 4.). 
I have been unable to detect anything approaching to the character of muscular 
fibre in the structure of the venous system in general. I have observed it, however, 
in the hepatic veins of the Seal ; and it doubtless exists in all animals subject to an 
arrest of the venous circulation around the heart. 
There yet remains a structure in the economy which presents an interest little in- 
ferior to that of the arterial system, I mean the iris. 
The tenacity of this membrane is greater than that of any other structure I have 
examined, so much so, as to render it exceedingly difficult of preparation under the 
dissecting microscope. 
When exhibited with the larger power it presents so much the character of the 
muscular fibre of organic life, that I feel almost inclined to associate it with that 
system. 
As regards the arrangement, I have less doubt than I have of the chemical compo- 
sition of the iris, which does not possess the semitransparent character of jibrine. 
