4'. M) ANATOMY IN A NUTSHELL. 



13. and 14. The inferior petrosal sinuses pass from the cavernous sinus 

 to the jugular foramen where they join the lateral sinus to make the internal 

 jugular vein. They arc situated where the posterior border of the petrous por- 

 tion of the temporal bone joins the basilar process of the occipital. 



15. The transverse siims (basilar sinus) connects the two inferior petrosal 

 sinuses. It passes over the basilar process of the occipital bone. 



The Muscles of the [schio-rectal Region. (Plate CCLXXI.) 



Corrugator cutis ani. — Description. — This muscle radiates from the ori- 

 fice of the anus and is a thin involuntary muscle. 



< Origin. — From the true skin externally from the anus. 



Insertion. — Into the submucous tissue internally. 



A.CTION. — To raise the skin in ridges around the anus. 



Nerve Supply Sympathetic nerve. 



Blood Supply. — Inferior hemorrhoidal. 



External sphincter ani. — Description. — This muscle surrounds the margin 

 of the anus and is elliptical in shape and thin and flat in form. It is about an 

 inch broad opposite tin 1 anus, and from its anterior to its posterior extremity 

 it is about three inches. It consists of two planes of muscular fibers. 



Origin. — From the tip of coccyx by a narrow tendon and from super- 

 ficial fascia in front of bone. 



Insertion. — Into the central tendons of perineum where it joins with the 

 Transversus perinaei, Levator ani. and Accelerator urinae. 



Action. -To close anus. 



Nerve Supply. — From the anterior division of the fourth sacral and in- 

 ferior lnemorrhoidal branch of the internal pudic nerve. 



Blood Supply. — Inferior hemorrhoidal from internal pudic. 



Internal sphincter ani. — Description. — This muscle surrounds the lower 

 part of the rectum for about an inch. It is composed of involuntary muscu- 

 lar fibers. 



A' HON.- -To close the amis. 



Nerve Supply. — Hsemorrhoi lal. 



Blood Supply.— Inferior hemorrhoidal from internal pudic. 



Levator ani. Description. — This muscle which is thin and broad is sit- 

 uated on the side of the pelvis. It helps to support the viscera in the pelvic 

 cavity. Sometimes the anterior portion of this muscle is separated from the 

 posterior portion by connective tissue, and then it is called the Levator pros- 

 tatas. 



Origin. From the posterior surface of the body and ramus of the os 

 pubis on the outer side of the symphysis. Also from the inner surface of the 

 spine of the ischium and from the fascia between these points. 



Insertion. I 1 I The posterior fibers are inserted into the sids of the apex 

 of the coccyx, and those fibers just in front of these mute with those of the 

 opposite side extending between the coccyx and the margin of the anus. (2) 

 The middle fibers, which form the greater portion of the muscle, are inserted 



