Chap. VII] MUSCULAR TISSUE 91 



substance. The fibre-cells are variously grouped in different 

 parts of the body ; sometimes crowded together in solid bundles 

 which are arranged in layers and surrounded by connective tis- 

 sue, as in the intestines ; sometimes arranged in narrow, inter- 

 lacing bundles, as in the bladder ; sometimes wound in single or 

 double layers around the blood-vessels ; and again running in 

 various directions and associated with bands of connective tissue, 

 they form large, compact masses, as in the uterus. 



Cardiac muscular tissue. — This variety of muscular tissue is 

 found only in the heart substance. It is involuntary, but is 

 striated, though not as distinctly as skeletal muscle. It is made 

 up of fibres which are short, contain just one nucleus, and no 

 sarcolemma. The fibres are grouped in bundles which are nearly 

 square, and fine fibrils from each cell help to hold the bundles 

 together. The bundles are really held together by connective 

 tissue, which forms a supporting framework in the heart, just as 

 it does in skeletal and visceral muscle. 



Stimuli. — This term is used to describe influences which stimu- 

 late muscle fibres. They may be chemical, mechanical, thermal, 

 electrical, or nervous. From the standpoint of physiology the 

 nervous impulse is the most important. 



Characteristics. — Muscular tissue is highly specialized and 

 exhibits irritability, contractility, extensibility, elasticity, and 

 tonicity. 



Irritability has been defined as the response of a tissue to a 

 stimulus. Nervous tissue and certain epithelial cells as well as 

 muscular tissue possess this property. The response of any 

 tissue to stimulation is to perform its special function, and in the 

 case of muscular tissue this response takes the form of contraction 

 and is known as muscular contractility. Each individual fibre 

 becomes shorter and thicker, and thus the whole muscle becomes 

 shorter and thicker. The function of the connective tissue frame- 

 work is passive and may be likened to that of a harness, through 

 which all the numerous contractile fibres are enabled to unite 

 their efforts. Contraction of the muscle tends to bring together 

 its two ends with whatever may be attached to them. Thus 

 the contraction of certain muscles of the arm will shorten the 

 muscles, shorten the distance between their ends, and flex the 

 forearm. 



