414 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



lytic state eventually results which is characterized by a general vas- 

 cular depression and may lead to the death of the animal. 



Very similar effects may be obtained by temporarily obstructing 

 the blood supply of the brain. In all experiments of this kind, how- 

 ever, it is advisable to deepen the narcosis by the administration of 

 curare, because in the non-curarized animal, the increased respiratory 

 movements, as well as the general muscular spasms which eventually 

 occur in the course of the asphyxia, must tend to raise the blood pres- 

 sure and to interfere with the effects of the vasoconstriction. It 

 need scarcely be emphasized that in the case of asphyxia, the constric- 

 tor agent may be either a lack of oxygen or a superfluity of carbon 

 dioxid. 



The Distribution of the Vasomotor Fibers. — The axons derived 

 from the cells of the vasomotor center descend in the cord and termi- 

 nate at different levels in the anterior horn of the gray matter. From 

 here connections are made with the sympathetic system by way of the 

 rami viscerales, but naturally, as these bridges exist only in the tho- 

 racic and sacral regions of the cord, the vasomotor outpourings must 

 necessarily be restricted to these spinal segments. It has also been 

 found that the cerebrospinal and sympathetic systems are connected 

 with one another by way of several of the cranial nerves, and hence, 

 it is possible that some of the vasomotor fibers leave the central 

 nervous system by way of these channels. At all events, it must be 

 concluded that while the control of the blood-vessels is in last analysis 

 a function of the cerebrospinal system, it is eventually transferred 

 to the autonomic or sympathetic system. 



After the spinal neurons have entered the sympathetic system their 

 impulses are conveyed to the more remote ganglia by secondary neu- 

 rons which in turn are connected with the blood-vessels of the thoracic, 

 abdominal and pelvic organs. The blood-vessels of the head are 

 reached by way of the ganglia of the thorax and the cervical sympa- 

 thetic which connects the latter with the superior cervical ganglion. 

 Obviously, therefore, the fibers conducting vasomotor impulses, are 

 typically autonomic and form such important paths as the greater 

 and minor splanchnic nerves, the nervi erigentes and the cervical 

 sympathetic. There are, however, many blood-vessels present in our 

 body which do not lie directly within the realm of sympathetic nerves, 

 but are innervated by cerebrospinal nerves. In this group belong 

 the blood-vessels of the anterior and posterior extremities. This 

 innervation is made possible by the fact that some of the fibers leave 

 the sympathetic system and enter the cerebrospinal nerves where they 

 intermingle with others pursuing a perfectly straight course from the 

 spinal gray matter to the periphery. These "recurrent" fibers form 

 the so-called gray rami viscerales. 



To summarize: the vasomotor fibers passing out from the chief 

 center, attain the first sympathetic ganglia by way of the anterior 

 roots of the cord, and the white rami viscerales (Fig. 222). Two or 



