THE CIRCULATORY SYSTEM 1(17 



Nerves. — These are derived from both cerebro-spinal (vagus) 

 and sympathetic systems (cervical ganglia) and consist of both 

 medullated and non-medullated fibres. Sympathetic ganglion cells 

 are distributed in groups throughout the myocardium, the largest, 

 lying in the epicardium near the base of the heart, being known as 

 the cardiac ganglion or ganglion of Wrisberg. Among these cells 

 the nerve fibres form plexuses from which both motor and sensory 

 terminals are given oft' to the muscle. (For nerve endings in heart 

 muscle see page 446.) 



TECHNIC 



(i) The Heart. — Cut pieces through the entire thickness of the wall of one 

 of the ventricle's, care being taken not to touch either the serous surface or the 

 lining endothelium. Fi.x in formalin-^Iiiller's fluid (tcchnic 6, p. 7). Cut trans- 

 verse and longitudinal sections; stain with haimatoxylin-eosin (technic i, p. 20) 

 and mount in balsam. 



(2) Treat the entire heart of a small animal {e.g., guinea-pig or frog) in 

 the same manner as the preceding, making transverse sections through both 

 ventricles. 



(3) An entire heart, human or animal, may be fixed in the distended condi- 

 tion by filling with formalin- IMiiller's fluid under low pressure and then tying off 

 the vessels. The entire heart thus distended is placed in a large quantity'' of 

 the same fixative. 



De\elopment of the Circulatory System 



The blood-vessels and the heart begin their development separately and 

 afterward become united. Both are derived from mesoderm, but while the 

 heart develops within the embryo, the earliest blood-vessels and blood are 

 developed from extra-embrj-onic mesoderm. The earliest vessels to be formed 

 are the capillaries. These make their appearance in the mesoderraic tissue 

 near the periphery of the area vasculosa which surrounds the developing embryo. 

 Here groups of ceUs known as "blood islands" differentiate from the rest of 

 the mesodermic cells, appearing in the chick by the end of the first day of 

 incubation. The superficial cells of these islands become flattened to form 

 the endothelium, which is at this stage apparently a syncj'tium through which 

 the nuclei are scattered and showing no e\ddences of cell boundaries. This 

 endotheHal syncytium surrounds the remaining more central cells, from which 

 blood cells are developed. These represent the earliest blood-vessels. The 

 channels, which are at first unconnected, anastomose and give rise to a network 

 of channels which are the earliest capillaries. These develop rapidly in the 

 area vasculosa and some of them increase in size to become arteries and veins, 

 the smooth muscle and connective tissue of their walls being dift'erentiated 

 from the surrounding mesoderm. These grow toward, and finally into, the 

 embryo where they unite with the heart. In granulation tissue, and in new 

 growths in general, both normal and pathological, new blood-vessels apparently 

 develop as oft"-shoots from other vessels. These are at first solid extensions 



