198 STUDIES IN ADVANCED PHYSIOLOGY. 



the heart to contract more rapidly. In the case of the ves- 

 sels it is the vaso-constrictors. These nerves, as already 

 stated, when stimulated cause an increased contraction here. 

 In the case of the heart, as well as the vessels, these nerves 

 come directly from the sympathetic system, although they 

 take their real origin in the cerebro-spinal system. 



3. Both heart and vessels are supplied with nerves 

 which inhibit; that is, nerves, a stimulation of which 

 causes a reduction in the amount of contraction. In the 

 case of the heart this inhibitory nerve is the vagus, pneu- 

 mogastric, or tenth cranial nerve. In the case of the ves- 

 sels it is the vaso-dilators. As the vagus causes the heart 

 muscle to relax, so the dilators cause the arterial muscles 

 to relax. In both cases they are cerebro-spinal nerves and 

 reach their destination without the intervention of the sym- 

 pathetic system . 



4. The heart is supplied with an accessory nerve called 

 the depressor, which carries to the medulla sensations from 

 the cardiac muscle. While, in the case of the vessels no 

 such a typical sensory nerve exists, there are, of course, 

 distributed to them nerves of general sensation, by means 

 of which sensory impulses from them reach the brain. 



CHANGES IN THE CIRCULATION WHICH OCCUR AT BIRTH. 



It is evident that the lungs are not functional before 

 birth. The blood must, therefore, seek its fresh supply of 

 oxygen elsewhere. This has necessitated the blood taking 

 a somewhat different course up to the moment of birth from 

 the one it retains ever after. 



The foetus not only gets its nourishment, but its oxygen 

 supply also from a structure known as the placenta, or 

 "after-birth." This is a structure which grows out from 

 the foetus and intertwines itself closely with the uterine 

 wall; so closely, indeed, that the nourishment in the blood 

 flowing through the uterus may seep across into the foetal 

 capillaries of the placenta, and along with it the oxygen 

 carried by the arterial blood of the mother passes through 



