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. i6.) 

 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 

 urinogenital tubes, particular- 

 ly the bladder and the uterus. 

 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 



Fig. 17. Nerve-cell from the anterior grey 



column of the spinal cord (cf. Fig. 19) : d., 



dendrites; e.g., chromatophile granules; nr., 

 neurite. 



