2l6 THE CIRCULATORY SYSTEM. 



produced by vagus stimuli in at least some of the sympathetic neu- 

 rones situated in the heart, the neuraxes of which convey the im- 

 pulse to the heart muscle. The heart receives further nerve supply 

 through sympathetic neurones, the cell-bodies of which are situated 

 in the inferior cervical and stellate ganglia, the neuraxes of which 

 enter the heart as the augmentor or accelerator nerves of the heart. 

 The mode of ending of these nerve-fibers has not as yet been fully 

 determined. It may be suggested as quite probable that they ter- 

 minate on the dendrites of sympathetic neurones, the cell-bodies of 

 which are not inclosed by end-baskets of nerves reaching the heart 

 through the vagi, as above described. It is also possible that they 

 end directly on the heart muscle-cells. The cell-bodies of the 

 sympathetic neurones, the neuraxes of which form the augmentor 

 nerves, are surrounded by the telodendria of small medullated 

 fibers, forming end-baskets, which leave the spinal cord through the 

 anterior roots of the upper dorsal nerves. Besides the nerves here 

 described, nonmedullated nerves (whether the neuraxes of sympa- 

 thetic neurones, the cell-bodies of which are situated inside or out- 

 side of the heart has not been fully determined), form plexuses in 

 the walls of the coronary vessels, terminating, it would seem, on the 

 muscle-cells of the media (vasomotor nerves). 



2. THE BLOOD-VESSELS. 



A cross-section of a blood-vessel shows several coats. The 

 inner consists of flattened endothelial cells, and is common to all 

 vessels. The second varies greatly in thickness, contains most of 

 the contractile elements of the arterial wall, and is known as the 

 media, or tunica media. Its elastic fibers have in general a circular 

 arrangement and are fused at the inner and outer surfaces to form 

 fenestrated membranes, the lamina elastica interna and externa. 

 Outside of the media lies the adventitia or tunica externa, consist- 

 ing in the arteries almost entirely of connective tissue and in the 

 veins principally of contractile elements, smooth muscle-fibers. 

 Between the internal elastic membrane and the endothelial layer is 

 a fibrous stratum which varies in structure in the different vessels 

 of larger caliber. This is the subendothelial layer, or the inner 

 fibrous layer, and forms, together with the endothelium, the intima 

 or tunica intima. Bonnet (96), as a result of his own investigations, 

 suggests a somewhat different classification of the layers composing 

 the arterial wall. According to him, the endothelium alone con- 

 stitutes the intima. The elastic membranes, both internal and 

 external, together with the tissue lying between them, and that 

 between the internal elastic membrane and the intima, constitute 

 the media. The tissue layers outside the external elastic membrane 

 form the tunica externa (adventitia). 



(a) Arteries. In the great arterial trunks, such as the pulmo- 

 nalis, carotis, iliaca, etc., the tunica media possesses a very typical 



