KIDNEY AND SKIN AS EXCRETORY ORGANS. 787 



excitement. In such cases if the micturition is prevented, probably 

 by the action of the external sphincter, the bladder may sub- 

 sequently relax and the sensation of fullness and desire to micturate 

 pass away until the urine accumulates in sufficient quantity, or the 

 pressure is again raised by some circumstance which causes a reflex 

 contraction of the bladder. 



Nervous Mechanism. According to Langley and Anderson,* the 

 bladder in cats, dogs, and rabbits receives motor fibers from two 

 sources: (1) From the lumbar nerves, the fibers' passing out in the 

 second to the fifth lumbar nerves and reaching the bladder through 

 the sympathetic chain and the inferior mesenteric ganglion and 

 hypogastric nerves. Stimulation of these nerves causes a compara- 

 tively feeble contraction of the bladder. (2) From the sacral 

 spinal nerves, the fibers originating in the second and third sacral 

 spinal nerves, or in the rabbit in the third and fourth, and taking their 

 course through the so-called nervus erigens. Stimulation of these 

 nerves, or some of them, causes strong contractions of the blad- 

 der, sufficient to empty its contents. Little evidence was obtained 

 of the presence of vasomotor fibers. According to Nawrocki and 

 Skabitschewsky,t the spinal sensory fibers to the bladder are found 

 in part in the posterior roots of the first, second, third, and fourth 

 sacral spinal nerves, particularly the second and third. When these 

 fibers are stimulated they excite reflexly the motor fibers to the 

 bladder found in the anterior roots of the second and third sacral 

 spinal nerves. Some sensory fibers to the bladder may pass by 

 way of the hypogastric nerves. When the central stump of one 

 hypogastric nerve is stimulated it produces, according to these 

 authors, a reflex effect upon the motor fibers in the other hypo- 

 gastric nerve, causing a contraction of the bladder, the reflex oc- 

 curring through the inferior mesenteric ganglion. This observa- 

 tion has been confirmed by several authorities, but has been ex- 

 plained by Langley and Anderson as a pseudoreflex or axon reflex 

 (see p. 144). 



The immediate spinal center through which the contractions 

 of the bladder may be reflexly stimulated or inhibited lies, accord- 

 ing to the experiments of Goltz, in the lumbar portion of the cord, 

 probably between the second and fifth lumbar spinal nerves. In 

 dogs in which this portion of the cord was isolated by a cross-section 

 at the junction of the thoracic and lumbar regions, micturition still 

 ensued when the bladder was sufficiently full, and it could be called 

 forth reflexly by sensory stimuli, especially by slight irritation of 

 the anal region. This localization has been confirmed by others,}: 



*" Journal of Physiology," 19, 71, 1895. 



t"Archiv f. die gesammte Physiologie, " 49, 141, 1891. 



j See Stewart, "American Journal of Physiolog^, " 2, 182, 1899. 



