HAP. iv.] THE VASCULAR MECHANISM. 293 



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 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 carnese' 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 

 ordinary 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 morpho- 

 logically because the inner part of the cell round the nucleus is 

 unstriated, undifferentiated material 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 may be more or less distinctly 

 recognised. The vertebrate 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 as a rule enter the heart at the venous end of this tube, at 

 the sinus venosus, and pass on towards the arterial end, diminish- 

 ing in amount as they proceed, and disappearing at the aorta. 

 Connected 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 (according 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 



