582 VETERINARY PHYSIOLOGY 



Normally this is controlled by the central nervous system. 

 The bladder is supplied hy the j^elvic nerve, in which white fibres 

 run to the peripheral plexus, and by sympathetic fibres which 

 have their cell stations in the inferior mesenteric ganglion, from 

 which post-ganglionic fibres run to the bladder. In most 

 animals the former are chiefly augmentor, and the latter 

 inhibitory, but in some animals, e.g. the ferret, the reverse is 

 the case. Adrenalin causes relaxation or contraction, according 

 to whether the inhibitory or augmentor fibres run in the 

 sympathetics. 



The nerves are derived from a centre in the lumbar region 

 of the spinal cord which normally controls the peripheral 

 mechanism, and which may be reflexly excited by the passage 

 of some urine from the bladder into the urethra or in other 

 ways, e.g. in the dog by sponging the anus with warm water. 



In some cases of inflammation of the spinal cord {myelitis), 

 the increased activity of the centre may prevent the expulsion 

 of urine, while later in the disease, when the nerve structures 

 have been destroyed, the urine is not retained and dribbles 

 away on account of the absence of the tonic contraction of the 

 sphincter arrangement. 



The expulsion of the last drops of urine is carried out by 

 the rhythmic contraction of the bulbo-cavernous muscle ; while 

 the peristaltic contraction of the bladder wall is assisted by the 

 various muscles which press upon the contents of the abdomen 

 and the bladder. The horse micturates standing, but the ox 

 can do so while walking. 



In the young, micturition is a purely reflex act, and 

 in the dog it is perfectly performed when the spinal cord is cut 

 in the back. As age advances, the reflex mechanism comes to 

 be more under the control of the higher centres, and the activity 

 of the sphincters may be increased or abolished as circumstances 

 indicate. 



IV. Excretion by the Skin. 



The skin is really a group of organs, and some of these have 

 been already studied. {The structure of the skin and its 

 appendages must be studied ptractically.) 



(1) The Protective functions of the horny layer of epidermis. 



