MOTOR FUNCTIONS OF THORACICO-LUMBAR O UTFLO W 33 



FIG. 7. THE CONNECTOR FIBRES AND EXCITOR NEURONS OF THE SPHINCTER 

 SYSTEM OF INVOLUNTARY MUSCLES. 



The upper figure shows their arrangement in the mammal and the lower figure 

 their arrangement in the reptile (young crocodile). 



The connector neurons form two groups : an upper group rising from the last 

 dorsal and first three lumbar roots and all running to the superior mesenteric gan- 

 glion, S.M.G., which lies at the point of origin of the cceliac axis, C.A., from the 

 aorta, Ao., and a lower group rising from the second to fifth lumbar roots, L.2 to 

 L-5, and running to the inferior mesenteric ganglion, I.M.G., which is situated just 

 above the bifurcation of the aorta. 



The excitor neurons from the superior mesenteric ganglion innervate the ileo- 

 colic sphincter muscle, I.C.S., which lies at the junction of the small intestine, S.I., 

 and the large intestine, L.I. 



The excitor neurons in the inferior mesenteric ganglion supply in the mammal 

 the internal sphincter muscle, I.Sp., the sphincter of the bladder, S.B., and the 

 muscle of the urethra, M. U. 



The cavities of the bladder and large intestine are here entirely separate but 

 have been evolved from the arrangement shown in the lower figure. In the reptile 

 the cloaca is composed of a continuous tube divided into three portions, i. The 

 coprodeum, C.D., which corresponds to the large intestine of the mammal. 2. The 

 urodeum, U.D., which corresponds to the bladder cavity of the mammal and into 

 which the ureters, Ur., open, and 3. the proctodeum, P.O., which is the hindmost 

 chamber. A muscle corresponding to the internal sphincter of the mammal separates 

 the coprodeum from the urodeum, and is innervated by excitor neurons in the in- 

 ferior mesenteric ganglion. A similar muscle also separates the urodeum from the 

 proctodeum ; this corresponds with the sphincter muscle of the bladder and the 

 muscles of the urethra, and is innervated by excitor neurons in the inferior mesenteric 

 ganglion. 



