MUSCULAR TISSUE. 285 



MUSCULAR TISSUE. 



The muscular tissue is that by means of which the active movements of the body 

 are produced. It consists of fibres, which are for the most part collected into 

 distinct organs called " muscles," and in this form it is familiarly known as the flesh 

 of animals. These fibres, from a characteristic appearance which they exhibit under 

 the microscope, are usually known as " cross-striped " or " striated ; " they are 

 many of them under the control of the will, and are hence often spoken of as 

 " voluntary " muscles. Another kind of muscular tissue is disposed around the 

 blood-vessels and most of the hollow viscera, often forming a distinct coat or coats to 

 these. In this kind the fibres do not exhibit the same cross-striated appearance, 

 and they have therefore been termed in contradistinction "plain" or " non-striated " 

 muscular fibres. Most of these are entirely withdrawn from the control of the will, 

 and they are therefore also termed involuntary. The muscular tissue of the heart, 

 although having a cross-striated appearance, differs in many respects from that of 

 the skeletal muscles : it is therefore described separately under the term " cardiac " 

 muscular tissue. Muscular fibres are endowed with contractility, by virtue of which 

 they shrink or contract more or less rapidly under the influence of certain causes 

 which are capable of exciting or calling into play the property in question, and which 

 are therefore named stimuli. 



STRUCTURE OF CROSS-STRIATED OR SKELETAL MUSCLES. 



The skeletal muscular fibres are for the most part gathered into distinct organs 

 or muscles of various sizes and shapes, but most generally of an oblong form, and 

 furnished with tendons at each extremity, by which they are fixed to the bones. 



The fibres are in the first place collected into bundles, of greater or less thick- 

 ness, named fasciculi or lacerti (fig. 322). The fibres are parallel in the fasciculi ; 

 and the fasciculi extend continuously from one terminal tendon to the other, unless 

 in those instances, like the rectus muscle of the abdomen and the digastric of the 

 inferior maxilla, in which the fleshy part is interrupted by interposed tendinous 

 tissue. The fasciculi also very generally run parallel, and, although in many 

 instances they converge towards their tendinous attachment with various degrees of 

 inclination, yet in the voluntary muscles they do not interlace with one another. 



An outward investment or sheath of areolar tissue (epimysium) surrounds the 

 entire muscle, and sends partitions inwards between the fasciculi ; furnishing to 

 each of them a special sheath (perimysium).^ The areolar tissue extends also between 

 the fibres (endomysium), but does not afford to each a continuous investment, and 

 therefore cannot be said to form sheaths for them. Every fibre, it is true, has a 

 tubular sheath ; but this, as will be afterwards explained, is not composed of areolar 

 tissue. The perimysium contains elastic as well as white fibres ; but the elastic 

 element is found principally in its investing, as distinguished from its penetrating, 

 portion. In the endomysium numerous plasma-cells are found. The chief uses of 

 the areolar tissue are to connect the fibres and fasciculi together, and to conduct and 

 support the blood-vessels and nerves in their ramifications between the parts. The 

 relation of these different subdivisions of a muscle to each other, as well as the shape 

 of the fasciculi and fibres, is well shown in transverse section (figs. 322 and 323). 



1 The term perimysium has usually been employed to designate the external investment of the whole 

 muscle as well as the special sheath of each fasciculus. I have ventured, however, to introduce the 

 word epimysium for the general sheath, since the terms will then have a close analogy with those which 

 are applied to the connective tissue investments of a nerve (see p. 324). , 



u 2 



