In canine, as in human, atria cordis, the primary plexus is formed at the boundary between the epicardium and myocardium, which goes over into the secondary and tertiary plexus formed in the epicardium and myocardium. Minute fibres are also observed running into the endocardium. The incoming nerve fibres running into the primary plexus originate in the vagus and sympathetic nerves, the former consiting of non-medullated fibres, so that the distinction between sympathetic and parasympathetic fibres is not possible histologically. The distribution of nerve cell groups found in the plexus in the epicardium is also similar by human and canine hearts. Ganglia are mainly found in the dorsal walls of the atria and the entrance of the venae cavae into the heart, but sporadically in all other parts of the atria. In canine heart, as in human (Seto), nerve cell groups are found also in the secondary plexus in the myocardium. Nerve cells in the canine atria can be also classified into the cells of Dogiel's Type I and Type 11, of which the latter exist somewhat more abundantly by dog than by man. The nerve cells in canine atria are much smaller in size and simpler in construction than those in human heart, e.g., the pericellular connective tissue capsule being poorly developed, often even to total disappearance, the mantlecell-plasmodium being limited in size and the plasmodium-nuclei very much poorer in number. The number of nerve processes is also smaller than in human heart, especially with the nerve cells Type I, and the ramification and terminal formation of the short processes are much simpler. The plasmodium surrounding the nerve cells Type I is generally aligned in a uniform thickness, but as is the case in human heart, frequently the plasmodium is found especially developed at one pole of the cell. But the arrangement of short processes branching and ending therein is far simpler than in man. In nerve cells Type II, the number of short processes is rather smaller than in man, and the terminations thereof more rarely end in special plates. The terminations of incoming nerve fibres approaching the intragangliar nerve cells are represented by pericellular terminalreticulum, as in man, but these are much simpler than in man in construction. In dog, rather thick fibres are arranged in a comparatively dispersed interlacing mesh. The termination of vegetative nerve fibres is represented by the terminalreticulum in canine atria as in human atria. This comprises the three kinds of elements derived from the incoming sympathetic and parasympathetic fibres and the long processes from the nerve cells, consists of nerve cords formed by extremely minute fibrils arranged reticularly, ends never freely, and stands in tactile supply over any of the controled cells. A considerable number of medullated sensory fibres is seen running in canine atria. These end freely, and in form, can be classified into the unbranched endings, the simple branched endings and the endings Type I connected with the depressor reflex. These endings, however, are always much simpler in formation that in man. Their localization is also some-what different from that in man. Beside the above kinds of terminations, there are found branched sensory endings in the walls of the branches of aa. coronariae cordis. The unbranched terminations are found in the epicardium, the myocardium and the endocardium of the atria, but in special abundance in the myocardium. They end mainly sharply, but sometimes also upon completing an ansiform course. It is noteworthy that such endings are often formed close beneath the endothelial cell layers of the epi- and endocardium. In the simple branched endings consisting of 2 or 3 rami, the branches end generally sharply, but some end bluntly or either in a terminal body formed by fibril dissolution or describing a simple ansiform course.
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Jan 1, 1999
Imre Szecsödy 
Open Access