CHAP. VII.] CAPILLARIES AND NERVES OF MUSCLE. 



167 



Fig. 47. 



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 fragment of a frog's muscle, perfectly fresh, 

 be examined, a series of blood-particles will be 

 seen in the longitudinal capillaries. These 

 particles are compressed and elongated, 

 sometimes to a great extent, evidently by 

 the narrowness of the canal which contains 

 them. It may seem at first sight not doubt- 

 ful that in the living creature these elastic 

 blood-discs are similarly elongated in their 

 passage through the vessels of muscle, but 

 the admirable researches of Poiseuille will 

 perhaps serve to explain this appearance 

 without our being driven to suppose the pre- 

 sence of so formidable an obstacle to the 

 capillary circulation through these organs. 

 It is more probable that the contraction of 

 the vessels, and the compression of the 

 blood-discs, occur on the escape of some of 

 their contents being permitted by the cutting 

 off of the fragment for microscopic examina- 

 tion. The coats of the capillaries of muscle 

 consist of a simple diaphanous membrane, 

 in which a few irregular-shaped cyto-blasts Capillaries of asmaii fasciculus 



* of muscular fibres from the neck 



occur at infrequent intervals. 



It results from this description of the 



Capillaries Of muscle, that their number must relative size and direction of those 



to which the capillaries, here re- 



correspond nearly to that of the elementary presented, are distributed. 

 fibres; consequently, that the same amount of muscular tissue, 

 arranged as a large number of small fibres, would be supplied with 

 a larger absolute number of capillaries than if arranged as a small 

 number of large fibres. This difference of vascular supply is ex- 

 ceedingly remarkable, and will be reverted to in considering the 

 contractility of muscle. 



Of the Nerves of Muscle. Nerves being the appropriate channel 

 through which muscles are excited to contraction, we have now 

 to inquire into the manner in which the two tissues communicate 

 together. As far as is at present known, all muscles in the larger 

 animals have nerves distributed to them ; and, if we extend the 



of the Dog : a. Terminal twig of 

 the artery, v. Terminal twig of 

 the vein. p. Plexus of capillaries. 

 e. Elementary fibre, to shew the 



