MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



ORGANS OF CIRCULATION. 



The circulatory structures of the vertebrates consist of 

 fluids (blood and lymph) and the vessels in which they flow, 

 certain parts of which are specialized for the propulsion of the 

 contained fluid. The general characters of the blood and lymph 

 have already been described ; details will be given below when 

 necessary. On a priori grounds the lymph system is apparently 

 the older, but it will be more convenient to begin our account 

 with the blood-vascular system. In this we recognize in all 

 vertebrates a central muscular organ, the heart, which propels 

 the blood, vessels (arteries) which convey the blood to the pe- 

 ripheral portions, and other canals (veins) which bring it back 

 to the heart ; the extremities* of the arteries and veins being 

 connected by minute tubes, the capillaries. 



There is considerable evidence to support the view of 

 Biitschli, that the main trunks of the circulatory system are 



FIG. 192. Diagram of primitive condition of blood-vessels, a, transverse 

 vessels; d and v, dorsal and ventral vessels. 



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remnants of the segmentation cavity (p. 5) of the embryo, 

 which has otherwise become entirely obliterated by the ingrow- 

 ing mesoderm. The extension of the coelomic pouches towards 

 the middle line of the body above and below the alimentary 

 tract narrows the segmentation cavity in these regions into two 

 longitudinal tubes, the main circulatory trunks ; while from those 

 portions of the cavity between the myotomes arise semicircular 

 tubes uniting the dorsal and ventral tubes, the result being rep- 

 resented diagrammatically in Fig. 192. A part of the ventral 

 tube near the anterior end becomes specialized as the heart ; it 

 forces the blood forward through the anterior end of the ventral 

 tube which is known as the ventral aorta, then dorsally through 



