LYMPHATIC VESSELS. 137 



The nerves to the heart are the cardiac nerves from the cervical 

 sympathetic ganglia, and certain branches of the vagus. Together these 

 form the cardiac plexus with the associated cardiac ganglion [of Wrisberg] 

 at the base of the heart. Their fibers extend in plexuses containing 

 groups of cell bodies, over the dorsal walls of the atria, along the coronary 

 sulcus, and over the ventricles where, however, cell bodies are less numerous. 

 They lie in the epicardium but extend into the myocardium and appear 

 as bundles of non-medullated fibers. A few medullated fibers, supposed 

 to belong chiefly with sensory nerves, are found with them. Free sensory 

 endings, comparable with those in tendon, are numerous in the epicardium 

 and occur in the connective tissue of the other layers. They include 

 vagus fibers, which also terminate"' in baskets around the cell bodies 

 in the plexuses, but none are believed to pass directly to motor endings. 

 The motor terminations belong with ganglion cells in or near the heart. 

 Fibers from the cervical sympathetic ganglia may end in pericellular 

 baskets like the vagus fibers, or may pass directly to the muscles. Their 

 exact termination is not known. 



LYMPHATIC VESSELS. 



The lymphatic vessels are widely distributed through the body and 

 physiologically they are perhaps quite as important as the blood vessels. 

 They are however far less conspicuous. For this reason they are often 

 neglected by the student, who with some study should be able to find 

 them in a large proportion of the specimens examined. In a rabbit embryo 

 of 14 days and 18 hours, Fig. 161, the lymphatic system consists of several 

 spaces in close relation with the veins, lined with endothelium like that 

 of the blood vessels. The largest sac half encircles the internal jugular 

 vein and sends a considerable branch into the deep connective tissue of 

 the neck. Another large lymph space is near the renal veins; smaller 

 ones are with the mesenteric vessels, the azygos, and the external mammary 

 veins. An examination of younger embryos indicates that these lymphatic 

 vessels are detached branches of the adjacent veins. They are closed 

 endothelial tubes which send out ramifying branches into the subcutaneous 

 and other connective tissue, where they anastomose with one another or 

 end blindly. They do not anastomose with the blood vessels, which they 

 resemble, except for thinner walls and larger lumen. All of the lymphatic 

 structures in the rabbit of 14 days become connected with each other and 

 with similar new lymphatic vessels so as to form a system which empties 

 into the veins at two points, namely, into the subclavian veins near the 

 internal jugulars, on either side of the body. These openings have been 

 described as persistent original connections of the lymphatic vessels with 



