276 CARDIAC MUSCULAR TISSUE. [BOOK i. 



until at some little distance from the heart plain muscular fibres 

 only are found. Blood vessels are absent from the walls of the 

 auricles also. 



In the bulbus arteriosus, mixed up with much connective 

 and elastic tissue, are found fusiform fibres which close to the 

 ventricle are striated and form a thick layer, but at a certain 

 distance from the ventricle lose their striation, or rather become 

 mixed with plain muscular fibres, and form a thinner layer. 



152. In the mammal, both the ventricles and the auricles 

 are formed of bundles of muscular tissue, bound together by con- 

 nective tissue, and arranged more especially in the ventricles in 

 a very complex system of sheets or bands disposed as spirals, and 

 in other ways, the details of which need not detain us. In the 

 auricular appendices and elsewhere, the bundles form irregular 

 networks projecting into the cavities. 



The connective tissue binding the muscular fibres together, 

 unlike the corresponding connective tissue in the frog's heart, is 

 well supplied with blood vessels belonging to the coronary system. 

 This connective tissue forms on the inner surface of the cavities a 

 continuous sheet, the connective tissue basis of the flat epithelioid 

 cells of the endocardium, and on the outside of the heart the 

 visceral layer of the pericardium. 



The histological unit of these muscular bundles is neither a 

 fibre nor a fusiform fibre cell, butt a more or less columnar or 

 prismatic nucleated cell generally provided with one or more 

 short broad processes. The nucleus, which is oval and in general 

 resembles one of the nuclei of a striated fibre, is placed in about 

 the middle of the cell with its long axis in the line of the long 

 diameter of the cell. The cell body, which is not bounded by any 

 definite sarcolemma, is striated, though obscurely so, across, the 

 long diameter of the cell, the striations as in a skeletal muscle 

 fibre being due to the alternation of dim and bright bands. As in 

 the frog's heart granules are frequently abundant, obscuring the 

 striation, which indeed even in the absence of granules is never so 

 distinct as in the fibres of skeletal muscles. Such a cell is at each 

 end joined by cement substance to similar cells, and a row of such 

 cells constitutes a cardiac elementary fibre. Hence a cardiac fibre 

 is a fibre striated, but without sarcolemma, and divided by parti- 

 tions of cement substance into somewhat elongated divisions or 

 cells, each containing a nucleus. Many of the cells in a fibre have 

 a short broad lateral process. Such a process is united by cement 

 substance to a similar process of a cell belonging to an adjoining 

 fibre ; and by the union of a number of these processes, a number 

 of parallel fibres are formed into a somewhat close network. Each 

 bundle of the cardiac muscular tissue is thus itself a network. 

 These bundles are further woven into networks by connective 

 tissue in which run capillaries and larger blood vessels; and sheets 

 or bundles composed of such networks are arranged as we have 



