66 THE INVOL UNTAR Y NER VO US S YSTEM 



system of motor neurons which resemble those of the voluntary 

 system, in that their axons are all medullated, although the 

 muscles they supply are not striated. Now in the birds, the 

 corresponding muscles are striated, and Anderson has found 

 that in this case their motor cells are still in the ciliary ganglion, 

 but the axons of these cells are now large medullated fibres re- 

 sembling those to other striated muscles. Also the connector 

 fibres from the central nervous system to these cells are large. 



I conclude then that, when the motor neurons to a muscle 

 have once passed out from the central nervous system to form a 

 peripheral ganglion, they cannot go back into the central nervous 

 system even though the muscle take on the type of those belong- 

 ing to the voluntary system. 



I do not think this muscular system ought to be included in 

 either of the groups of involuntary muscles already considered. 

 The marked medullation of its motor nerves and the striation of 

 its fibres in birds place it in a separate category as something 

 intermediate between unstriped and striated muscle ; its nearest 

 ally is the endodermal musculature, but it is difficult to see how 

 it could have arisen from that group. It is significant in this 

 respect that Dale could not find any effect upon the pupil by 

 the injection of acetyl-choline. 



The innervation of another muscle of the eye may now be 

 discussed. The dilatator muscle of the mammalian pupil forms a 

 layer of radially arranged muscle fibres below the posterior limiting 

 membrane of the iris, which are so covered with pigment that for 

 a time it was disputed whether they were muscle fibres at all, and 

 whether the dilatation of the pupil upon stimulation of the cervical 

 sympathetic was not brought about through the relaxation of the 

 fibres of the sphincter muscle by stimulation of inhibitory nerve 

 fibres. Langley and Anderson however settled this question by 

 cutting a radial strip of the iris and showing its shortening upon 

 stimulation ; he also caused a deformation of the shape of the iris 

 upon stimulation at the edge of the sclerotic in the intact eye, es- 

 pecially at the places where the long ciliary nerves enter the iris. 

 These peculiar, pigmented, radially arranged cells therefore are 

 muscular in nature. Their motor cells are situated in the superior 

 cervical ganglion and their motor fibres reach the muscle by way 

 of the long ciliary nerves. I conclude that the dilatator muscle 

 belongs to the system of dermal muscles, its motor neurons are 



