ANAL CANAL. 



1231 



Very often the contained vein presents an enlargement, or a knob-like tortuous plexus 

 in the lower part of the column ; below this the plexus is continued down external to the 

 mucous membrane of the 

 lower zone of the anal canal 

 into the anal veins. This 

 portion has accordingly 

 been described as the 

 hsemorrhoidal zone of the 

 anal canal. Sometimes the 

 columns are very indistinct ; 

 occasionally no trace of them 

 can be found, although in 

 the foatus they are usually 

 well marked. 



Anal canal\ I 



Anal valves 



FIG. 962.- 



-THE INTERIOR OF THE ANAL CANAL AND LOWER PART 

 OF RECTUM, 



ends, 

 usual. 



The columns were more numerous in this specimen than 



Anal Valves. If a 



probe is passed down- 

 wards along the groove 

 which separates two ad- 

 jacent columnse rectales 

 (Fig. 962), its point will 

 usually catch in a small 



Crescentic fold which joins Showing the columnse rectales, and the anal valves between their lower 



the lower ends of the two 



columns. These little folds, 



which resemble in miniature the segments of the semilunar valves of the heart, 



are the anal valves. They project inwards and upwards, and behind each is found 



a little pocket-like sinus (sinus rectalis). 



Processus Yermiformis Superior haemorrhoidal vessels 



^L _ 



___--- Root of pelvic mesocolon 



--^Cower end of pelvic colon 



nternal spermatic vessels 

 enito-femoral nerve 

 External iliac vessels 



Obturator nerve 

 Umbilical artery 

 Obturator vessels 

 Ureter 



Umbilical artery 7 



Pelvic plexus of nerves 



- Rectum 



Obt. vessels and nerve 



Ureter 



Pelvic plexus of nerves an 



hsemorrhoidal veins . .,,. 



Levator am 



Anal canal 

 External sphincter 



FIG. 963. DISSECTION OF THE RECTUM FROM THE FRONT IN A SPECIMEN HARDENED BY FORMALIN 



INJECTION. 

 The front wall of the pelvis has been removed, and the bladder, prostate, and seminal vesicles taken away. 



These valves were first described by Morgagni. Kecently the view has been advanced by 

 Ball that they are the remains of the embryonic cloacal or anal membrane ; and he explains the 

 production of " painful fissure of the anus " by the tearing down of one of them during defaecation 

 by hardened masses of faeces. 



The epidermis is continued in a thin and modified form from the exterior up along the anal 

 canal as far as the superior end of the columnse rectales ; and the view is pretty generally held that 



