262 PHYSIOLOGY. 



ing back again. Like the arteries, they are supplied with 

 nerves from Hie great sympathetic nerve. 



Fig. 4. 





Portion of an artery, showing the several coats of which it is com- 

 posed, separated from each other. 1. The internal or serous coat ; 2. 

 The middle or fibrous coat; 3. The external or cellular coat. 



14. The capillary system, so called from the vessels being 

 small, like hairs, presents two modifications. \The first con- 

 sists of little tubes, furnished with proper coats or walls, 

 which are the termination of the arteries and the commence- 

 ment of the veins. But a part of these small vessels do not 

 terminate in veins, but in the very substance of the flesh, 

 and the organs themselves. These are even smaller chan- 

 nels than the first, and permit only a single globule of blood 

 to pass out at a time. They are probably formed by the 

 fine arterial vessels, gradually losing their proper coats, and 

 becoming confounded with the cellular tissue/ 



15. In the capillary system, the functions of secretion, 

 nutrition, absorption and calorification, or the production of 

 animal heat, are performed. It is often divided into two 

 sections, the general and the pulmonary. The first has al- 

 ready been described. The other exists only in the lungs, 

 and connects the pulmonary arteries and veins. In this the 

 blood is changed from venous to arterial. 



16. The account which has been given of the circulation 

 applies to the human species, mammalia and birds. ^Reptiles 

 have but one heart, containing one ventricle and one or two 



