316 PHYSIOLOGY CHAP. 



the temperature exceeds certain limits, and lengthen when it 

 drops. But if these muscles are electrically stimulated when they 

 have almost or quite reached the maximal thermal contraction, 

 their reaction is di Hi-rent. 



The sphincter of the rabhit (whether atropiuised or not) 

 responds to each stimulation by a marked contraction, followed 

 by a still more marked relaxation, which takes the writing-point 

 below the original zero line. Not infrequently, on stimulating 

 with strong induced currents, the muscle lengthens directly 

 without any previous contraction. This is constantly the case on 

 stimulating with strong galvanic currents. 



The sphincter of the cat's pupil brhavrs somewhat differently. 

 It is very difficult \\ith gulvunic excitation to obtain any contrac- 

 tion of it ; generally it lengthens in proportion to the intensity of 

 the stimulus. It is rare to find the elongation preceded by a 

 small and transient emit ruction. 



Griinhagen recognised tbut the lengthening of the sphincter 

 muscle is not a fatigue-effect, und gave the name of clvmjation to 

 this iii'tire relaxation in agreement with the theory we ha\e 

 brought forward elsewhere (Vol. III. p. 30) to the eil'ect that huth 

 contraction and expansion are the effects of two opposite physio- 

 logical processes. 



The fact observed by Steinach (1882) and fully confirmed later 

 on by Guth (1901), that the sphincter muscle of certain animals 

 reacts directly to light by contracting, is of physiological interest. 

 Consequently it seems in every respect to be a muscle endowed 

 with peculiar physiological properties and specific excitability. 



The dilatator is quite distinct in its anatomical and physio- 

 logical characters from the sphincter. According to Grynfeld 

 (1899) it is a continuous layer of fibrils radially disposed in front 

 of the pigment cells of retinal origin which cover the posterior 

 surface of the iris. It seems to be a variety of contractile tissue, 

 similar to smooth muscle tissue, from which, however, it differs 

 by the fusion of its contractile substance into a continuous layer. 

 Its physiological value is considerably less than that of the 

 sphincter, as is admitted by all ophthalmologists. 



The motor fibres that innervate the muscles of the iris are 

 carried by the cerebral and the sympathetic nerves (Vol. III. 

 p. 363). 



The constrictor fibres of the iris run with the oculo-motor 

 nerve. They pass thence to the ciliary ganglion, enter into con- 

 nection with its cells, and run on in the short ciliary nerves to 

 the sphincter of the pupil. The relation of the motor paths of 

 the sphincter and of the ciliary muscle to the cells of the ciliary 

 ganglion was clearly demonstrated by Langley and Anderson 

 (1894). Yet the ciliary ganglion does not seem to be in any way 

 a peripheral centre for its afferent and efferent nerves. 



