MUSCLE. 339 



according to my observations, incorrect. Thus, the muscular 

 substance of the heart does not form fibres at all, but consists 

 simply of fibrilla?, which agree in every respect with those of 

 other involuntary muscles, save in their transverse striation : 

 thus they have the same considerable diameter : they are, in 

 like manner, abundantly nucleated (see Plate XLI. Jig. 3.), 

 and they have the same arrangement, interlacing with each 

 other, and not forming fibres included in a sarcolemma, as is 

 the case with the voluntary muscular tissue. The transverse 

 striae, too, have not the deep and permanent character belong- 

 ing to the fibrillaB of ordinary striped muscle, as is evinced 

 by the fact that acetic acid effaces all vestige of striation. - 



It thus appears that the muscles of the heart agree in 

 structure much more closely with that of other involuntary 

 muscles, which is contrary to what is generally supposed, 

 than they do with that of the voluntary muscles. 



Thus then the structure and the function of the muscles of 

 the heart are in accordance, and not in antagonism, as is usu- 

 ally conceived, the single point of difference between its fibrilla3 

 and those of other involuntary muscles consisting in the feeble 

 transverse striation, the presence of which evinces a somewhat 

 higher degree of development, as well as a greater power of 

 contractility. 



Structure of Striped Muscular Fibre. The striped muscle 

 consists of fibres : each of these is included in a distinct enve- 

 lope, termed Sarcolemma, and is made up of a number of 

 lesser fibres or fibrilhe. (See Plate XLIL fig. 1.) 



The fibres vary very considerably in size, not merely in 

 different animals, but also according to the age of an animal, and 

 even in a single bundle of the same animal, some of them being 

 three or four times larger than others, and the smaller being 

 usually adherent to the larger fibres ; a fact which has refer- 

 ence to the development of muscular tissue, and which will 

 be explained when we arrive at the consideration of that 

 portion of our subject. (See Plate XLIL fig. 4.) The dif- 

 ference in the size of the fibres according to age is very re- 

 markable, those of the foetus being several times smaller than 

 the fibres of the adult. (See Plate XLIL fig. 1. and Plate 

 XLIII. fig. 1.) 



