96 



PRINCIPLES OF ANIMAL BIOLOGY 



Cardiac muscle is found only in the heart of vertebrate animals. It 

 contains fibrils somewhat resembling those of striated muscle, and has 

 cross striations which these fibrils confer on it. However, the strands 

 of heart tissue interconnect in a network, and there is little or no blocking 

 off of the protoplasm into cells. The heart is thus practically a large 

 syncytium (page 66). 



The actions of the three kinds of muscle are very different. Smooth 

 muscle is capable of only relatively slow movement. It is not directly 

 subject to the will, hence is sometimes called involuntary muscle; but 

 this is not a distinctive designation, since the heart is also free from 



Fig. 86. — Curve illustrating the course of a single muscle twitch. 



conscious control, and even striated muscle sometimes acts involuntarily. 

 Striated muscle acts very strongly and very rapidly; and since its move- 

 ment is regularly initiated by act of will, it is known as voluntary muscle. 

 Heart muscle acts without control of the will, as do other vital organs. 

 Its rhythmic action can be maintained for long periods after removal of 

 the organ from the body, as determined by a mechanism to be described 

 in a later chapter. Because of its syncytial nature, waves of stimulation 

 pass rapidly over the whole heart, and the organ tends to act as a single 

 unit. 



Muscle Contraction. — In the living animal, contraction is stimulated 

 only by nerve impulses, though in laboratory experiments artificial 

 stimuli can be given. A single nerve cell may govern only a few muscle 

 cells, or as many as 150. The group of muscle cells controlled by one 

 nerve fiber constitutes a motor unit. It is characteristic of motor units 

 that, if they (contract at all, they do so to their fullest capacity, in accord- 

 ance with the all-or-none law already stated (page 53). Since muscles 

 are made up of many motor units, some contracting, others usually not, 

 an entire muscle may experience many degrees of contraction. How 

 many motor units act depends on the intensity of the nerve stimulus, a 

 strong stimulus activating many of them, a \veak stimulus few. 



A single stimulus to a striated muscle results in a single quick twitch 

 of the muscle. If tlic muscle is attached to a movable pointer, which 



