THE CIRCULATORY SYSTEM. 



103 



necessity for such close arrangement. The deeper fibrous layers 

 of the pericardium and of the endocardium receive numerous capil- 

 laries, a few being also found within the chorda tendineae and the 

 valves. 



The lymphatics of the heart are very numerous. They form 

 a comprehensive system, embracing the lymph-spaces occupying 

 the clefts between the muscle-fibres and the rich net-works of 

 more definite channels extending within the pericardium and endo- 

 cardium, including the valves. These two sets of lymph-radicles 

 communicate but sparingly and pursue largely independent courses. 

 Lymphatic vessels also accompany the branches of the coronary 

 arteries. 



The rich nervous supply of the heart is derived from the coro- 

 nary plexuses, and includes numerous medullated fibres coming from 

 the pneumogastric, as well as the non-medullated sympathetic fibres 

 proceeding from the cervical ganglia. Numerous microscopic gan- 

 glia are found along the course of the larger nerve-trunks accom- 

 panying the branches of the coronary arteries, especially in the 

 longitudinal interventricular and in the auriculo-ventricular furrows. 

 Many additional small groups of ganglion-cells occur within the 

 muscular tissue associated with the fibres supplying the intimate 

 structure. The nerves and the blood-vessels are covered by the 

 visceral pericardium. 



The development of all parts of the circulatory apparatus 

 takes place within the mesoderm ; while possessing a common 

 origin, the blood-vessels and the heart, however, develop in- 

 dependently, and, for a 

 time, are distinct and dis- 

 connected. The earliest 

 blood-vessels appear near 

 the periphery of the vascu- 

 lar area, outside the limits 

 of the proper body of the 

 embryo ; later and second- 

 arily they extend centrally 

 and unite with the primitive 

 heart and those parts of the 

 large trunks which have 

 been formed coincidently 

 within the embryo. 



The mesodermic elements within certain tracts near the periphery 

 of the vascular area undergo proliferation, which results in the pro- 

 duction of deeply staining densely nucleated areas known as the 

 blood-islands of Pander ; these are the direct progenitors of the 



Developing capillary blood-vessels within the omentum 

 of young rabbit : a, a, elongated protoplasmic processes 

 connecting the walls of the newly-formed capillary (c) 

 with the angioblastic connective-tissue corpuscles (i). 



