286 NERVOUS INFLUENCES AFFECTING INTESTINAL MOVEMENTS. 



The active peristalsis necessary to cause defecation may be favored 

 and to a certain extent excited, partly by pressure, partly by short 

 voluntary movements of the external sphincter and the levator ani 

 muscles, whereby the my enteric plexus of the lower portion of the large 

 intestine is stimulated mechanically, with the result that active peris- 

 taltic movements of the large intestine are soon set up. The expulsion 

 of feces is favored by active, voluntary abdominal pressure, principally 

 with inspiratory depression of the diaphragm. The soft parts of the 

 pelvic floor are forced downward conically with a strong effort at stool, 

 whereby the anal mucous membrane, which coincidently becomes filled 

 with venous blood, is at times everted. It is the function of the levator 

 ani muscle (Figs. 108 and 109) voluntarily to elevate the soft parts 

 forming the pelvic floor and thus, in elevating the anus, in a measure 

 to slide it over the descending column of feces. At the same time it 

 prevents relaxation of the soft parts of the pelvic floor, particularly the 

 pelvic fascia. As the fibers of both levator ani muscles converge down- 

 ward, and mix with those of the external anal sphincter, they coinci- 

 dently aid the sphincter when energetic contraction takes place, as they 

 bear approximately the same relation to the anus that the strings of a 

 tobacco-pouch bear to its opening. When the desire for stool is 

 marked the closure of the anus can be made more secure by pressure 

 from without through forcible rotation of the thighs outward and the 

 action of the gluteal muscles. 



During the normal interval between evacuations of the bowel, the 

 feces appear to descend only to the lower extremity of the sigmoid 

 flexure. From this point to the anus the rectum normally is usually 

 free from feces. The strong circular fibers of the muscularis, which 

 Nelaton termed the third anal sphincter, appear, by their contraction, 

 to arrest the further advance of the fecal matter. 



NERVOUS INFLUENCES AFFECTING THE INTESTINAL MOVE- 

 MENTS. 



The automatic center for the movements of the intestinal canal is 

 the greatly developed myenteric plexus, which is embedded between 

 the longitudinal and circular layers of the muscular coat. It is this 

 that is responsible for the movements that continue for some time in 

 an excised portion of intestine, just as they occur in the heart. 



This plexus, constituted mainly of non-medullated nerves, distributes fibers 

 that, after again forming a network, pass to the unstriated muscle-fibers. The 

 cells of the plexus possess an axis-cylinder process and several protoplasmic pro- 

 cesses. Nerve-fibers pass through the mass of ganglia, while others surround 

 the ganglion-cells with their extremities. Special nerve - plexuses, containing 

 ganglia, are found upon the blood-vessels and lymph- vessels of the intestinal 

 wall. 



When this center is free from all stimulation, the intestine remains in 

 a state of rest, resembling the apnea that occurs with absence of stimu- 

 lation of the medulla oblongata. This occurs during intra-uterine life, 

 as it does also with respect to respiration, in consequence of the large 

 amount of oxygen in the fetal blood. This condition may be termed 

 intestinal rest aperistalsis. It is observed also during sleep, perhaps 

 in consequence of the greater amount of oxygen in the blood. 



The circulation through the intestinal vessels of blood containing 



