ISCHIO-RECTAL FOSSA. LITHOTOMY. 1043 



endangered. The part of the intestine now under observation rests against 

 the conjoined levatores ani, the coccyx, and the sacrum. 



The lower end of the rectum receives small arteries on each side from the 

 pudic ; but its principal artery (the superior hsemorrhoidal, the continuation 

 of the inferior mesenteric, p. 412,) descends behind the organ and ends 

 in branches about three inches from the anus, which enter the gut and 

 anastomose in loops opposite the internal sphincter. The veins, like those 

 of the abdomen generally, are without valves. These vessels are very 

 liable to enlarge and become varicose ; and this condition is constantly 

 associated with or even forms a great part of the disease known as 

 haemorrhoids. 



Ischio-rectal fossa. On each side of the rectum between it and the ischiul 

 tuberosity is contained a considerable quantity of fat, the space which it 

 occupies being named the ischio-rectal fossa. This hollow extends back- 

 wards from the periuasum to the great gluteal muscle ; it is bounded on 

 the inner side by the levator ani as this muscle descends to support the 

 intestine, and on the opposite side by the obturator fascia and muscle 

 supported by the hip-bone. At the outer side and encased in a sheath of 

 the obturator fascia is the pudic artery with the accompanying veins and 

 nerve ; and small offsets from these cross the fossa to supply the lower end 

 of the rectum. The pudic artery, it will be observed, is about an inch 

 above the lower surface of the tuber ischii, and at the same time, by its 

 position under that prominence of the bone, it is protected from injury by 

 incisions directed backwards from the perineum ; but in front of this part 

 (in the peiinseum proper), inasmuch as the vessel lies along the inner 

 margin of the subpubic arch, it is here liable to be wounded when the 

 deeper structures of the periuaeum are incised. 



The fossa is narrowed as it reaches upwards into the pelvis ; such nar- 

 rowing of the space is the necessary result of the direction of the levator 

 ani, which drops inwards from the fascia on the side of the pelvis, and thus 

 limits the fossa at its upper end. 



LATERAL OPERATION OF LITHOTOMY. 



The intention of the operation, as it U usually performed, is to remove 

 a calculus from the urinary bladder by an opening made through the 

 perinseum and the prostatic part of the urethra. The incisions to attain 

 this end are commonly made on the left half of the perinseum : because 

 this side is most convenient to the right hand of the operator ; but if the 

 surgeon should operate with the left hand, then the opposite (right) side 

 of the perinceum would be most convenient. 



The position at which the perinagum is to be incised requires careful 

 consideration. For, if the necessary incisions should be made too near the 

 middle line of the body, the bulbous enlargement of the corpus spongiosurn 

 urethras and the rectum are liable to be wounded ; and if, on the other 

 hand, the perinaeum should be divided towards its outer boundary (the 

 pubic arch), there is a risk of wounding the pudic artery where that vessel 

 has reached the inner edge of the bone. The incisions are therefore to be 

 made through the area of the small perineal spnce in such manner as to 

 avoid both its sides. Again, as to the length to which the several struc- 

 tures are to be incised : The integument and the subcutaneous fatty layer 

 must be divided with freedom, because, first, the skin does not admit of 

 dilatation during the removal of the foreign body ; and, secondly, exten- 

 sive incisions through the structures near the surface facilitate the egress 



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