220 MUSCULAR FIBKE-CELLS OF THE ARTERIES. 



nnmeroTis single cells may be seen and followed from 

 one end to the other without difficulty. (Fig. 1, 

 plate V, page 217.) 



263. Muscular fibre-cells of tlie arteries. — But of 



all the forms of unstriped muscle, that which encircles 

 the small arteries and ramifies over the coats of the 

 veins, is, in many respects, the most interesting, for 

 by its influence the calibre of the small vessels is 

 altered, and the amount of blood to flow through the 

 capillaries of a particular tissue in a given time de- 

 termined, and its movement regulated. If the 

 pressure employed in injecting the vessels artificially 

 be very gradually increased, the smaller arterial tubes 

 may be distended, so as to separate very slightly from 

 one another the encircling muscular fibre cells ; and 

 in fortunate specimens prepared in glycerine, I 

 have succeeded in gently tearing asunder the vessel, 

 so as to display not only each individual muscular 

 fibre cell with its bioplasm, but the nerve fibres dis- 

 tributed to it. A good example of this is seen in Fig. 

 2, plate V, page 217, and another in Fig. 4, plate 

 XVI, § 12. The distribution of the nerves to these 

 muscular fibres, and the arrangement of the mechanism 

 by which the blood flow is varied, will, however, be 

 further considered in my last lecture. 



264. Striated or strii»ecl muscle. — The fibres of 

 voluntarij or striped muscle differ from those of the 

 involuntary muscular tissue, in many particulars. 

 They exhibit transverse as well as longitudinal mark- 

 ings, and easily cleave or are split up in these direc- 

 tions. The fibres vary very much in size and general 

 arrangeraent in different animals, and in different 

 muscles of the same animal. The elementary fibres 

 of one muscle may be less than the ■5- J^-th of an inch 

 in width, while those of other muscles attain a 

 diameter of as much as the yV*^ of an inch. The 

 elementary fibres of insect muscle exhibit tlie general 



