3IOVE3IEXTS OF THE ALIMENTARY CANAL, ETC. 389 



a co-ordinated series of efferent impulses is sent out to the various muscles, 

 has not been satisfactorily determined. It has been shown that the portion 

 of the nervous system through which the reflex is effected lies in the me- 

 dulla. But it has been pointed out that the muscles concerned in the act 

 are respiratory muscles. Vomiting in fact consists essentially in a simul- 

 taneous spasmodic contraction of expiratory (abdominal) muscles and inspi- 

 ratory muscles (diaphragm). It has therefore been suggested that the reflex 

 takes place through the respiratory centre, or some part of it. This view 

 seems to be opposed by the experiments of Thumas, 1 who has shown that 

 when the medulla is divided down the mid-line respiratory movements con- 

 tinue as usual, but vomiting can no longer be produced by the use of emetics. 

 Thumas claims to have located a vomiting centre in the medulla in the imme- 

 diate neighborhood of the calamus scriptorius. Further evidence, however, 

 is required upon this point. The act of vomiting may be produced not only 

 as a reflex from various sensory nerves, but may also be caused by direct 

 action upon the medullary centres. The action of apomorphia is most easily 

 explained by supposing that it acts directly on the nerve-centres. 



Micturition. — The urine is secreted continuously by the kidneys, is car- 

 ried to the bladder through the ureters, and is then at intervals finally ejected 

 from the bladder through the urethra by the act of micturition. 



Movements of the Ureters. — The ureters possess a muscular coat consisting of 

 an internal longitudinal and external circular layer. The contractions of this 

 muscular coat are the means by which the urine is driven from the pelvis of the 

 kidney into the bladder. The movements of the ureter have been carefully 

 studied by Engelmann. 2 According to his description the musculature of the 

 ureter contracts spontaneously at intervals of ten to twenty seconds (rabbit), the 

 contraction beginning at the kidney and progressing toward the bladder in the 

 form of a peristaltic wave and with a velocity of about twenty to thirty milli- 

 meters per second. The result of this movement should be the forcing of the 

 urine into the bladder in a series of gentle rhythmic spirts, and this method of 

 filling the bladder has been observed in the human being. Suter and Maver 3 

 report some observations upon a boy in whom there was ectopia of the bladder 

 with exposure of the orifices of the ureters. The flow into the bladder was 

 intermittent and was about equal upon the two sides for the time the child 

 was under observation (three and a half days). 



The causation of the contractions of the ureter musculature is not easily 

 explained. Engelmann finds that artificial stimulation of the ureter or of a 

 piece of the ureter may start peristaltic contractions which move in both direc- 

 tions from the point stimulated. lb' was not able to find ganglion-cells in the 

 upper two-thirds of the ureter, and was led to believe, therefore, that the con- 

 traction originates in the muscular tissue independently of extrinsic or intrinsic 

 nerves, and that the contraction wave propagates itself directly from muscle- 



1 Virchovfs Archiv fur pathologische Anatornie, etc., L891, Bd. 123, S. II. 



" PfiUger l 8 Archivfiir diegesammte Physiologie, 1869, Bd. ii. B. 243; Bd. iv. S. 33. 



3 Archiv fiir exper. Patholog'x- vmd Pharmakologie, 1893, Bd. 32, S. 241. 



