108 MUSCULAR TISSUE. 



exciting or calliDg into play the property in question, and which arc 

 therefore named stimuli. A large class of muscles, comprehending 

 those" of locomotion, respiration, expression, and some others, are 

 excited by the stimulus of the will, or volition, acting on them through 

 the nerves ; these are therefore named " voluntary muscles," although 

 some of them habitually, and all occasionally, act also in obedience to 

 other stimuli. There are other muscles or muscular fibres which are 

 entirely withdrawn from the control of the will, such as those of the 

 heart and intestinal canal, and these are accordingly named "involun- 

 tary." These two classes of muscles differ not only in the mode in 

 which they are excited to act, but also to a certain extent in their 

 anatomical characters; and on this account we shall consider the 

 structure of each class separately. 



OF THE STRUCTURE OF VOLUNTARY MUSCLES. 



The voluntary muscular fibres are for the most part gathered into 

 distinct masses 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 two attached extremities of a muscle are named, in anatomical 

 descriptions, its origin and insertion ; — the former term being usually 

 applied to the attachment which is considered to be most fixed, although 

 the rule cannot be always applied strictly. The fleshy part is named 

 the belly, which in some cases is interrupted in the middle or divided 

 into two by a tendon, and then the muscle is said to be biventral or 

 digastric ; on the other hand it may be cleft at one end into two or 

 three porticms, in which case it is named bicipital or tricipital. 



The muscular fibres are collected into packets or bundles, of greater 

 or less thickness, named fasciculi or lacerti (fig. G-1), and the fibres 

 themselves are commonly described as consisting of much finer threads, 

 visible by the aid of the microscope, which are termed muscular fila- 

 ments, fibrilla?, or fibrils (fig. G5, c). The fibrils run parallel with 

 each other in the fibres, and 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. 



Sheath. — An outward investment or sheath of areolar tissue (some- 

 times named /lerimij.sium) surrounds the entire muscle, and sends par- 

 titions inwards between the fasciculi ; furnishing to each of them a 

 special sheath. The areolar tissue extends also between the fibres, 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 tnltular 

 sheath ; but this, as will be afterwards explained, is not derived from 

 the areolar tissue. The tissue of the sheath is composed of elastic 

 (yellow) as well as of white fibres ; but the elastic element is found 

 principally in its investing (as distinguished from its penetrating) 

 l)ortion. The chief uses of the areolar tissue are to connect the fibres 

 and fasciculi together, and to conduct and support the blood-vessels and 



