58 PRINCIPLES OF VETEBINARY SCIENCE 



THE INVOLUNTARY MUSCLES 



Instead of being under the direct control of the will, as are 

 the voluntary muscles, the involuntary muscles act independently. 

 Their properties may be contrasted with the voluntary as follows : 



Location. — The walls of all hollow organs such as the stomach, 

 intestines,, bladder, bronchial tubes, arteries, and veins contain 

 involuntary muscles. Since they occur only in soft organs, 

 they are known as visceral muscles. 



Color. — Their color is pale or light red. 



Volume. — They are found in relatively small masses. 



Form. — They conform to the form of the walls of the organs 

 in which they are located. 



Shape. — They are always found in thin sheets or tubes. 



Structure. — Involuntary muscles are composed of spindle- 

 shaped cells about }£ 5 o inch in length, each cell having a 

 single nucleus (Fig. 2). These cells are grouped together so 

 as to give the appearance of a smooth layei, hence the name 

 smooth or unstriped muscle. The sarcolemma is thin but tough 

 and elastic and adapted to the surface of each cell. 



Nerve Supply. — This comes directly from the sympathetic 

 nervous system instead of from the central nervous system, as in 

 the case of voluntary muscles. Numerous ganglion cells are 

 distributed throughout involuntary muscles. 



Blood Supply. — Nutriment is supplied by many very minute 

 blood-vessels. 



Arrangement. — They are always arranged in two layers in the 

 walls of the soft organs, viz., an outer longitudinal and an inner 

 circular layer (Fig. 25). 



Names. — When there are any given, the names correspond 

 to those of the organs in which the muscles are located. 

 THE HEART MUSCLE 



Cardiac muscle is midway between the voluntary and involun- 

 tary muscles. It resembles the former in being composed of 

 striped fibers and the latter in not being under the control of the 

 will. The grouping of the individual fibers, however, differs 

 somewhat from that of typical striped muscle. The nerve 

 supply' is in part from the vagus nerve of the central nervous 

 system and in part from the sympathetic system; impulses from 

 the former inhibit the heart muscle contractions, while 'those 

 from the latter augment or accelerate them. 



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