516 



MUSCLE. 



arranged as a few long fibres (as in the sar- 

 torius), or as many short ones (as in the 

 masseter). In the former case its con- 

 tractions would be characterized by their ex- 

 tent, in the latter by their power; for, cateris 

 paribus, the extent is as the length, the power 

 as the thickness. 



The terms origin and insertion are employed 

 with great convenience in ordinary anatomical 

 language to denote the more fixed and the more 

 moveable attachments of muscles. Tn human 

 anatomy general consent has sanctioned their 

 use, and even, with few exceptions, their par- 

 ticular application to each muscle in the body, 

 although this assignment is in many cases arbi- 

 trary, in consequence of its being impossible to 

 determine which attachment is the more fre- 

 quently the fixed one. 



The arrangement of the fibres in the heart 

 has been already fully treated of in this work. 

 (See HEART, Fibres of.) 



In the muscular coat of the alimentary canal, 

 of the bladder, and uterus, the unstriped fibres 

 are disposed, as in the heart, so as to enclose 

 a cavity, but without having, as in that organ, 

 any point at which they can be said to com- 

 mence or terminate. In the alimentary tube 

 they are arranged in two laminae, the respective 

 fibres of which take a course at right angles to 

 each other. In the bladder the arrangement is 

 reticulate. The elementary fibres form sets 

 of variable thickness, which at numerous 

 points send off detachments to join neigh- 

 bouring bundles, whence has sprung the notion 

 that the fibres are branched. It is mani- 

 festly, however, the sets of them only that are 

 branched, the unstriped like the striped fibres 

 being invariably simple from end to end. In 

 the uterus the disposition of the fibres is essen- 

 tially similar, calculated to allow of great 

 variety in the capacity of the cavity they 

 encircle. 



Of the areoltir tissue of muscles. This tissue 

 is much more abundant in the voluntary than 

 in the involuntary muscles. In the former it 

 forms an external investment, which sends 

 septa into the intervals between the larger and 

 smaller packets of fibres, and thus enables 

 them to move in some degree independently 

 of one another. The density of thesf> general 

 and partial sheaths is proportioned to the 

 amount of pressure to which the organ may be 

 subject, as is exemplified in the superficial 

 muscles of their back, and in those superficial 

 muscles generally where a fibrous aponeurosis 

 does not perform the same office. The areolar 

 tissue does not usually clothe every individual 

 fibre from end to end, giving it a cellular 

 sheath, except in cases where the elementary 

 fibres are of large dimensions. Besides the 

 protection the areolar tissue affords to the mus- 

 cular fibres, it admits of motions between 

 them. But it must also serve the important 

 office of limiting undue motions of one part 

 of a muscle on another part, by its form- 

 ing a connecting bond between neighbouring 

 bundles. But a principal use of it appears to 

 be that of furnishing a resisting nidus in which 

 the delicate vessels and nerves can traverse the 



interstices of the fibres, and by which they 

 can be protected from hurtful dragging during 

 the unequal and oscillating movements of the 

 fibres of a voluntary muscle during its slate of 

 activity. This idea is supported by the fact 

 that scarcely any areolar tissue exists in the 

 heart or in the unstriped muscles generally. 

 In the heart, though the contraction is power- 

 ful, it is instantaneous or nearly so, and there- 

 fore probably more uniformly diffused, so that 

 neighbouring fibres must be less moved on one 

 another than in the more sustained contraction 

 of a voluntary muscle. Moreover the mutual 

 intertwining of even the elementary fibres in 

 this organ is, in many parts of it, so intricate, 

 as to contribute much to their mutual support. 

 And in the other involuntary muscles, the con- 

 tractions are slowly and evenly progressive 

 along the fibres of the same set. 



Of the bloodvessels of muscles. The arteries 

 and veins of muscles commonly run together, 

 and most of the arterial branches, to within 

 two removes from the capillaries, are accom- 

 panied by two vena comites. They invariably 

 pass more or less across the direction of the 

 fibres, divide and subdivide, first in the in- 

 tervals between the larger sets, then between the 

 smaller sets, till the ultimate twigs insinuate 

 themselves between the fibres composing the 

 smallest bundles, and break up into their capil- 

 lary terminations. In this course the vessels 

 supply the areolar tissue, their own coats, and 

 the attendant nerves. The capillary plexus of 

 the areolar membrane consists of irregular but 

 pretty equal-sized meshes, and contrasts strongly 

 with that of the muscular tissue itself. The 

 proper capillaries of muscle are quite charac- 

 teristic in their arrangement, so that a person 

 who has once seen them can never afterwards 

 mistake them. They consist of longitudinal 

 and transverse vessels, the longitudinal always 

 following the course of the elementary fibres, 

 and lying in the intervals between them, the 

 transverse being short communications placed 

 at nearly equal distances between the longitu- 

 dinal ones, and crossing nearly or quite trans- 

 versely over or under the fibres. The manner 

 in which the longitudinal vessels are placed 

 in relation to the fibres, is seen in fig. 286, 

 where I have represented them as they are 

 seen on a transverse section. They usually 

 occupy the interstice between three or more 

 fibres, but sometimes also the space between 

 the contiguous surfaces of two fibres. The 

 length of the longitudinal vessels does not 

 usually exceed the twentieth of an inch ; in other 

 words the terminal twigs of the artery and vein 

 pertaining to the same capillary are seldom 

 further than that apart. The length of the 

 transverse anastomosing capillaries necessarily 

 varies with the thickness of the fibres over 

 which they pass (fig. 299). The diameter of 

 the capillaries of muscle varies like that of 

 others with the size of the blood- particles of 

 the animal. It is, however, only just sufficient 

 to allow of the particles to pass. If a frag- 

 ment of a frog's muscle, perfectly fresh, be 

 examined, series of blood-particles will be 

 seen in the longitudinal capillaries. These 



