NORMAL HISTOLOGY. 



CHAPTER IV. 



THE MUSCULAR TISSUES. 



CONTRACTILITY is possessed, to a certain degree, in common by 

 all cells rich in active protoplasm ; the distinguishing characteristic 

 of muscular tissue, however, is that this property is so conspicuously 

 developed in highly specialized structures, and that the contractions 

 take place along definite lines in limited directions alone. Con- 

 tractile tissue or muscle occurs in two principal forms : (i) as the 

 non-striated, smooth, or vegetative muscle, usually beyond 

 the control of the will, and hence called involuntary, and (2) as the 

 striated, striped, or animal muscle, which, being influenced by 

 volition, is known as voluntary. 



The sharp differences separating the two groups of muscle in man 

 and the higher animals cannot be regarded as fundamental, since in 

 the embryonal condition of these higher forms temporarily, and in 

 the adult form of the lower types permanently, the striped and non- 

 striated varieties of muscle depend upon the degree of specialization 

 rather than upon inherent differences. It is a suggestive fact that 

 long before the cells forming the embryonal heart show indications 

 of differentiation into muscle-tissue the contractions of the organ 

 have commenced. The association of the striped fibres with response 

 to the will and, on the contrary, of the plain tissue with involuntary 

 action must be, likewise, only provisionally accepted, since in some 

 animals the development of marked striae never takes place in the 

 voluntary fibres. Standing between and connecting the extremes 

 of these groups is the cardiac muscle of the higher vertebrates, in 

 which the fibres are striated, although beyond the control of the will. 



NON-STRIATED OR INVOLUNTARY MUSCLE. 



Non-striated, smooth, or involuntary muscle, while never occurring 

 in large individual masses, enjoys a wide distribution ; its principal 

 localities are 



1. The Digestive Tract: the muscularis mucosae from oesophagus 

 to anus and the delicate bundles of mucosa and villi ; muscular tunic 

 from the lower half of oesophagus to anus. 



2. The Accessory Digestive Glands : in the large excretory ducts 

 of liver, pancreas, and some salivary glands ; also in the gall-bladder. 



3. The Urinary Tract : in the capsule and the pelvis of kidney, 

 ureter, bladder, and urethra. 



