1 24 THE INVOL UNTAR Y NERVOUS S YSTEM 



both the cases cited the gland cells are very largely surrounded 

 by unstriped muscle, so that, as already mentioned, the contrac- 

 tion of such muscle must squeeze out a secretion already formed. 

 It is not however possible to explain all glandular secretion 

 brought about by stimulation of a nerve as due to the squeezing 

 action of contractile tissue. 



The classical example of nervous action upon gland cells is 

 the action of the chorda tympani on the secretory activity of the 

 submaxillary gland. It has been shown conclusively that stimu- 

 lation of this nerve, which is a branch of the facial nerve, causes 

 a marked flow of saliva along Wharton's duct from the gland, and 

 further that it consists of small medullated fibres which connect 

 with cells of the submaxillary ganglion. This ganglion is situ- 

 ated close to the gland, and sends secretory fibres into the gland 

 itself. In every respect the innervation is so similar to that of 

 involuntary muscles, as to compel us to look upon these secretory 

 nerve cells as belonging to the same system as the motor nerve 

 cells of the involuntary system. 



Further, the evidence seems to me conclusive that the fibres 

 from these cells are true secretory nerves, for, apart from the 

 actual flow of the secretion, marked histological differences occur 

 in the gland cells according to whether they have been caused 

 to secrete by nerve stimulation or are at rest. Also Barcroft 

 has shown that active metabolism of the gland cells does take 

 place on stimulation of the chorda tympani, for a largely increased 

 amount of oxygen is taken in, in consequence of the nerve stimu- 

 lation. Stimulation of the chorda causes also a large increase of 

 the flow of blood through the gland, and it might possibly be 

 argued that this increased flow causes the metabolism of the gland 

 cells and not any direct action of secretory nerves. Barcroft and 

 Muller have however shown that the injection of yohimbin will 

 cause a ten-fold increase in the blood-flow through the gland 

 without any secretion of saliva, and that such increase does not 

 cause any measurable change in the oxygen used by the gland. 

 The gland after such treatment responds to stimulation of the 

 chorda tympani with a good secretion of saliva accompanied by 

 a seven-fold increase in the oxygen taken up. We must there- 

 fore include true secretory nerve cells and nerve fibres in the 

 involuntary nervous system, and the chorda tympani and similar 

 nerves must be classed with the other cerebro-spinal nerves as 



