36 ANATOMY OF THE RABBIT. 
possess an insertion after the fashion of ordinary muscles; and in 
some cases, as in the intrinsic muscle of the tongue or the so-called 
orbicular or sphincter muscles, both origin and insertion may be 
absent. 
Involuntary muscle is distinguished by its white or greyish 
coloration and by its smooth or homogeneous appearance. It forms 
characteristic layers in connection with visceral organs or with the 
skin, and is thus much less individual than the voluntary muscles 
in its relations to particular parts. It forms the muscular portion 
(muscularis mucosae) of the mucous tunic of the alimentary 
canal, and also a separate muscular tunic lying in the outer 
portion of its wall (Fig. 16.) 
In the muscular tunic the 
fibres are arranged in both 
circular and _ longitudinal 
directions. Involuntary mus- 
cle also forms a small con- 
stituent of many organs, such 
as certain glands, in which 
contractility is not a chief 
function. It forms a large 
constituent of the wall of the 
mes Wi che ea or fe iene ore urinogenital tubes, particular- 
dendrites; c.g., chromatophile granules; nr., ly the bladder and the uterus. 
aie In association with elastic 
connective tissue it is an important constituent of the walls of the 
bloodvessels. 
Although there is an underlying community of structure in the 
walls of the bloodvessels, the two chief types of vessels, arteries and 
veins, present conspicuous differences, both in functional behaviour, 
and in their appearance in the dead animal. The differences are 
largely the result of differences in the relative amounts of the above- 
mentioned constituents. The arteries are thick-walled, elastic 
tubes, which, under the force of blood from the heart, first become 
greatly expanded, and then gradually contract, so that the blood 
is forced into the smaller capillary vessels. The veins on the other 
hand are thinner-walled, less elastic vessels, through which the 
blood is forced largely through the pressure from behind. In the 

