CHAP, iv.] THE VASCULAR MECHANISM. 277 



said in a complex manner both in the auricle and ventricle. 

 Hence the muscular substance of the mammalian heart is, at 

 bottom, an exceedingly complex network, the element of which is a 

 somewhat branched nucleated striated cell. It may be remarked 

 that the ' musculi pectinati ' of the auricle and the ' columnae 

 carneae ' of the ventricle suggest the origin of the mammalian 

 heart from a muscular labyrinth like that of the frog's ventricle. 



At the commencement of the great arteries this peculiar 

 cardiac muscular tissue ceases abruptly, being replaced by the ordi- 

 nary structures of an artery, but the striated muscular fibres of 

 the auricle may be traced for some distance along both the venae 

 cavae and venae pulmonales. 



Under the endocardium are frequently present ordinary plain 

 muscular fibres, and in some cases peculiar cells are found in this 

 situation, the cells of Purkinje, which are interesting morphologi- 

 cally because the body of the cell round the nucleus is ordinary 

 clear protoplasm while the outside is striated substance. Plain 

 muscular fibres are said also to spread from the endocardium for a 

 certain distance into the auriculo-ventricular valves. 



153. The Nerves of the Heart The distribution of nerves in 

 the heart varies a good deal in different vertebrate animals, but 

 nevertheless a general plan is more or less evident. The verte- 

 brate heart may be regarded as a muscular tube (a single tube, if 

 for the moment we disregard the complexity of a double circulation 

 occurring in the higher animals) divided into a series of chambers, 

 sinus venosus (or junction of great veins), auricle, ventricle and 

 bulbus (or conus) arteriosus. The nerves (with the exception of 

 a small nerve which in some animals reaches the heart by the 

 aorta) enter the heart at the venous end of this tube, at the sinus 

 venosus, and pass on towards the arterial end, diminishing in 

 amount as they proceed, and disappearing at the aorta. Con- 

 nected with the nerve fibres thus passing to the heart are groups, 

 smaller or greater, of nerve cells. These like the nerve fibres 

 are most abundant at the venous end (appearing on the nerve 

 branches before these actually reach the heart), as a rule become 

 fewer towards the arterial end, and finally disappear, so that (ac- 

 cording to most observers) at the bulbus (conus) arteriosus they 

 are entirely absent. 



These collections of nerve cells or ganglia may be arranged in 

 groups according to their position. In many lower vertebrates 

 there is a distinct ring or collar of ganglia at the junction of the 

 sinus venosus with the auricle, where the primitive circular 

 disposition of muscular fibres is maintained; and there is a 

 similar ganglionic collar at the junction of the auricle with the 

 ventricle, where also there is similarly retained a circular dis- 

 position of the muscular fibres forming the so-called canalis auri- 

 cularis. And indeed in all vertebrates two similar collections of 

 ganglia are more or less distinctly present. There are ganglia 



