484 PRINCIPLES OF GENERAL PHYSIOLOGY 



THE CEREBRAL CIRCULATION 



Before passing on to the consideration of certain questions relating to the 

 innervation of the viscera and the blood vessels, a few words may be said as to 

 the blood supply of the brain. As will be seen later, there is no adequate evidence 

 that the cerebral vessels have any vasomotor control ; the importance of the brain 

 is such that its circulation is regulated by the whole of the rest of the body, 

 which is caused to accommodate itself, by constrictor and dilator nerves, to the 

 needs of the brain (Bayliss and Hill, 1895). 



A further interesting fact is the way in which, in the higher vertebrates, the 

 main arterial supply is formed by cross connections between all the four arteries 

 taking part, so that the interruption of one source does not deprive the brain of 

 blood. This arrangement, known as the "circle of Willis," is shown in Fig. 149, 

 which is a copy of one of Willis' plates (1680). 



This plate is of interest for two other reasons. It shows the numeration of the cranial 

 nerves with which the name of Willis is associated, and it was drawn for him by his friend, 

 Christopher Wren, who, as is said in the preface, " eruditissimis snis manibus delineare non 

 fuit gravatus," ''did not think it too much trouble to draw with his skilful hands" many of 

 the plates in the book. 



THE SYMPATHETIC AND OTHER PARTS OF THE AUTONOMIC 



SYSTEM 



The relation of the nervous supply of the viscera to that of the muscular and 

 skeletal (somatic) components of the organisms was first made clear by the work 

 of Gaskell (1886 and 1889). Gaskell's work on the innervation of the heart led 

 him to see that the sympathetic nervous system, which consists of a chain of 

 ganglia united by nerves with a particular region of the spinal cord, was not a 

 separate nervous system, interchanging fibres with the cerebrospinal system, as had 

 been taught, but was formed by certain fibres given off by the thoracic and upper 

 part of the lumbar regions of the cord. These nerves consist of fine medullated 

 fibres and form the white rami communicantes of the sympathetic ganglia. The 

 grey rami were shown by Gaskell to be in reality peripheral nerves given off by 

 cells in the sympathetic ganglia and distributed to the blood vessels of the spinal 

 cord or its membranes. The white rami, then, form synapses in the ganglia with 

 other neurones which have non-medullated axones, some of which pass to the spinal 

 cord as grey rami, others are distributed to the viscera and blood vessels through- 

 out the organism. The fibres of the white rami, however, do not all lose their 

 medullary sheath in the sympathetic chain, but some of them do so in other, 

 more peripheral, ganglia. 



Further investigation, with this clue, led to the recognition of two other 

 similar outflows of efferent, ganglionated, visceral nerves, one in the cranial nerves, 

 the other in the sacral region. These latter were originally known as nervi 

 errigentes, but, since they supply the bladder and rectum and other organs of the 

 pelvis also, Gaskell called them " pelvic splanchnics " to show their uniformity 

 with the abdominal splanchnics. These three outflows are separated by two gaps, 

 where the nerve plexuses for the anterior and posterior limbs are found. 



Langley (1898, p. 241), in order to obviate the confusion which might be 

 caused by calling the sympathetic nerves which supply the skin, " visceral," 

 proposed the name "autonomic," suggested to him by Professor Jebb. "The word 

 implies a certain degree of independent action, but exercised under the control of 

 a higher power." " The autonomic nervous system means the nervous system of 

 the glands and of the involuntary muscles ; it governs the ' organic ' functions 

 of the body." 



It is necessary to be quite clear that the autonomic system includes the sympathetic, since 

 some writers abroad use the name as applying to all the visceral nervous system other than the 

 sympathetic, speaking of sympathetic and autonomic. This practice leads to confusion, as 

 well as being JMijustified by facts. 



Although, as we shall see in Chapter XXIV., the sympathetic system is distinguished by 

 certain peculiarities not shared by the rest of the autonomic system, it is convenient to have 

 a name for the whole of the visceral system of nerves in contradistinction to the somatic 

 svstem. 



