668 SPECIAL HISTOLOGY. 



derably the thicker; it is more fibrous towards the exterior, presenting, 

 towards the interior, numerous fine, elastic networks, which are imme- 

 diately covered with one or two layers of tessellated epithelium. Very 

 numerous elastic networks of the same kind are found also in the inner 

 thin lamella, which is, partly, very intimately united with the muscular 

 substance, and partly, especially in the sulci, separated from it by com- 

 mon adipose tissue, which, moreover, not unfrequently forms a subserous 

 fatty layer, extending almost over the entire heart. The vessels present 

 the same conditions as elsewhere ; and with respect to the nerves, tAvigs 

 from the phrenic and recurrent branch of the right vagus have been 

 demonstrated in the outer lamella of the pericardium (Luschka). 



The muscular fibres of the heart are red and transversely striated, 

 but differ in many respects from those of the voluntary muscles. The 

 individual fibres themselves are, on the average, about Jd more slender 

 (0'004 O'Ol of a line), frequently more distinctly striated in the longi- 

 tudinal than in the transverse direction, and pretty readily divisible 

 into fibrils and minute particles (" sarcous elements," Bowman); their 

 sarcolemma is very delicate, or even wholly inappreciable ; and in the 

 fibres, there almost uniformly occur minute fatty granules, which, with 

 the nucleus, are frequently disposed in a series in the axis of the fibre, 

 and, where the muscular tissue is degenerated, appear most usually to 

 be excessively multiplied, and also colored. Much more, however, than 

 by these characters, is the muscular tissue of the heart distinguished by 

 the intimate union of its elements, which, except on the internal surface 

 of the organ, not only never form manifestly distinct bundles, being 

 everywhere in close apposition with each other, and separated only by 

 a scanty connective tissue, but, as was discovered by Leeuwenhoek,* 

 275. an( ^ I a ^ so nave f<> un( l (vid. p. 108), are directly united 

 together in their elements. These anastomoses of the 

 muscular fibres, which are a universal attribute of the 

 cardiac muscular tissue, are effected in the human and 

 mammalian heart generally, chiefly, by short, oblique, 

 or transverse, usually small fasciculi, and are extremely 

 numerous, so that, in many places in the ventricles and 

 auricles (whether universally I know not), numerous in- 

 stances of them are presented in every minute portion. 

 Besides these, there also exist true divisions or fibres, by 

 which the thickness of separate portions of muscle may 

 be rendered more considerable than it was originally. 



The course of the muscular fibres of the heart is extremely complex, 

 and a general outline only of it can here be given. The muscular struc- 

 tures of the ventricles and of the auricles are completely distinct from 



FiG. 275. Anastomosing primitive fasciculus from the human heart. 

 * [See note, p. 108. TRS.] 



