CARDIAC MUSCLE. 



55 



The blood-vessels supplying involuntary muscle, meagre in comparison 

 with those of the striped muscle, are guided in their distribution by the 

 septa of connective tissue, in which the larger vessels run. These give 



off minute branches that terminate in 

 capillary networks which extend be- 

 tween the primary bundles of fibre-cells. 

 Numerous lymphatics likewise follow the 



,' 



FIG. 68. Portion of injected intestitial wall. 



FIG. 67. Section of uterus, showing bundles^ showing arrangement of blood-vessels supply- 

 of involuntary muscle cut in various directions. ing involuntary muscle ; upper layer longitudi- 

 X 220. nally, lower transversely cut. X 50. 



larger septa of connective tissue. The nerves supplying involuntary muscle 

 are sympathetic fibres. The larger trunks form plexuses, closely associated 

 with microscopic ganglia, from which delicate twigs pass between the bun- 

 dles of fibre-cells. Their ultimate relation with the contractile tissue is 

 described with the Nerve-Endings (page 86). 



CARDIAC MUSCLE. 



The contractile tissue constituting the greater bulk of the heart repre- 

 sents a type of muscle which, so far as histological differentiation is con- 

 cerned, stands between the simpler smooth muscle and the highly complex 

 striated tissue. The striking peculiarity of cardiac muscle, namely its re- 

 ticular arrangement, is referable to embryonic conditions. The mesenchyma, 

 from which the heart-muscle develops, for a time exists as a protoplasmic 

 reticulum that contains irregularly distributed nuclei but is without cell- 

 boundaries. This tissue corresponds, therefore, to a syncytium. As the 

 syncytial network becomes more compact, owing to the increasing width of 

 its trabeculae with corresponding diminution of the intervening spaces, deli- 

 cate contractile threads, the myo-fibrils, make their appearance within the 

 reticulum and extend lengthwise through the trabeculae, without regard to 

 the limits of the future cell-areas. Notwithstanding the differentiation of 

 the greater part of the syncytium into contractile fibrillse and the conversion 

 of the spongy embryonal tissue into the compact tissue of the heart-wall, 

 evidences of the primary reticular arrangement are seen in the characteristic ' 

 networks formed by the adult cardiac muscle. 



