The Nervous System. 275 



of the pupil ; it is found low down in the cord, between 

 the cervical and dorsal portion. 



The ano-spinal centre, found in the lumbar portion of 

 the cord, controls the act of defecation ; it would appear 

 to be highly developed in herbivora, which possess the 

 power of bringing it into play not only when the body is 

 at rest, but during movement. The functions of the ano- 

 spinal centre are rather complex, inasmuch as it has to 

 maintain the tone of the sphincter, and also, on receiving 

 the needful sensory impressions from the bowel, to relax 

 the sphincter, contract the wall of the intestine, and cause 

 a contraction likewise of the abdominal muscles. 



The vesico-spinal centre, which also exists in the lumbar 

 portion of the cord, governs micturition ; its action is like 

 that of the ano-spinal centre. 



The erection centre lies in the lumbar cord; the genito- 

 spinal centre contains the nervous apparatus employed in 

 the emission of semen ; the parturition centre is situated in 

 the lumbar part of the cord. 



Vaso-motor centres, both constrictor and dilator, are 

 found throughout the cord; they are principally under 

 the control of similar centres in the medulla, but may act 

 independently. Sweat centres are probably closely con- 

 nected with the vaso-motor centres. Trophic centres for 

 the nutrition of the tissues also exist in the cord ; destruc- 

 tion of parts by ulceration follows injury of the trophic 

 nerves. 



Centres for the maintenance of muscular tone exist in 

 the cord; by means of them the muscles are kept taut 

 and ready for immediate action. 



Medulla Oblongata. 

 Situated at the top of the spinal cord, and forming the 

 connection between it and the brain, is the medulla 

 oblongata. It is composed of white and grey matter, but 

 not arranged with the regularity found in the cord. The 

 columns of the latter are continued into it, and give rise to 

 certain columns in the medulla larger and more prominent 



18—2 



