166 LOCOMOTION. [CHAP. VIT. 



activity. This idea is supported by the fact that scarcely any 

 areolar tissue exists in the heart, or in the unstriped muscles gene- 

 rally. In the heart, though the contraction is powerful, it is instan- 

 taneous, or nearly so, and therefore probably more uniformly dif- 

 fused, 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 elemen- 

 tary fibres in this organ is in many parts of it so intricate, as to 

 contribute much to their mutual support; and, in the other invo- 

 luntary muscles, the contractions are slowly and evenly progressive 

 along the fibres of the same set. 



Of the Blood-vessels of Muscles. The arteries and veins of mus- 

 cles commonly run together : and most of the arterial branches, to 

 within two removes from the capillaries, are accompanied by two 

 vena comites. They invariably pass more or less across the direc- 

 tion of the fibres, divide and subdivide, first in the intervals 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 capillary 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 characteristic in their 

 arrangement, so that a person, who has once seen them, can never 

 afterwards mistake them. They consist of longitudinal and trans- 

 verse 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 dis- 

 tances between the longitudinal ones, and crossing nearly, or quite, 

 transversely over or under the fibres. The manner in which th( 

 longitudinal vessels are placed in relation to the fibres, is seen ii 

 fig. 36, represented as they are seen on a transverse section. 

 usually occupy the interstice between three or more fibres, but 

 sometimes also the space between the contiguous surfaces of twc 

 fibres. The length of the longitudinal vessels does not usually ex- 

 ceed the twentieth of an inch ; in other words, the terminal twij 

 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 ove 

 which they pass (fig. 36, B, a 1 } . 



