THE A BDOMEN 877 



Lymphatic Vessels of Rectum, Anal Canal, andAnus^{i) Rectum. 



■ — The h-mphatics of the upper part of the rectum pass partly with 

 the superior hemorrhoidal artery to the inferior mesenteric group 

 of the pre-aortic glands, and partly to the rectal glands. The 

 lymphatics of the lower part of the rectum pass with the middle 

 hemorrhoidal artery to the internal iliac glands. 



(2) Anal Canal. — The lymphatic vessels of the upper part of the anal 

 canal, above the anal valves, pass to the internal iliac glands. Those 

 of the lower part of the anal canal, below the anal valves, pass to the 

 pubic, and, it may be, the superficial femoral, or saphenous glands. 



(3) Anus. — The Ijinphatic vessels of the integument of the anus 

 pass to the pubic, and, it may be, the superficial femoral or saphenous 

 glands. 



Lymphatic Glands of Rectum. — The rectal glands, which are small 

 and few in munber, he behind the rectum. They receive as afferents 

 some of the IjTnphatic vessels of the upper part of the rectum, and 

 their efferent vessels pass to the lateral sacral glands. 



Nerves. — ^These are partly sympathetic and partly spinal. The 

 superior hemorrhoidal artery conducts to the rectvun and anal 

 canal the superior hemorrhoidal sympathetic plexus, which is an 

 offshoot from the inferior mesenteric plexus, that in turn coming 

 from the aortic plexus. The middle hemorrhoidal arteries conduct 

 the middle hemorrhoidal sympathetic plexuses, which are offshoots 

 from the pelvic plexuses. The spinal fibres are derived from the 

 third and fourth sacral nerves (sometimes also the second), and 

 they belong to the pelvic splanclmics. They are fiurther derived 

 from the upper two or three lumbar nerves, aU in the manner 

 described in connection with the innervation of the bladder. The 

 fibres from the pelvic splanchnics carry motor impulses to the 

 longitudinal muscular fibres of the rectmn, and inhibitory impulses 

 to the circular fibres ; whilst the sympathetic fibres are motor as 

 regards the circular fibres, and inhibitory as regards the longi- 

 tudinal fibres. The anal canal, close to the external sphincter, 

 receives tAvigs from the inferior hemorrhoidal branch of the pudic 

 nerve. 



Development of the Reetam, Anal Canal, and Anas. 



The terminal portion of the hind-gut presents a blind dilatation, lined 

 with entoderm, which is Ccilled the cloaca (entodermal cloaca). This chamber 

 is freely connected with (i) the hind-gut, (2) the uro-genital ducts, and 

 (3) the allantoic diverticulum, all of which open into it in early embr^-onic 

 life. The ventral wall and terminal extremity of the cloaca are formed by 

 a membrane, derived from the caudal part of the primitive streak, called the 

 cloacal membrane. This membrane consists of two layers, external and 

 internal. The external layer consists of ectoderm, and the internal layer of 

 entoderm, in which respects it is homologous with the bucco -pharyngeal 

 membrane at the cephahc end of the fore-gut. 



The cloacal membrane separates the cloaca from a superficial depression 

 called the cloacal fossa, or ectodermal cloaca, which is divided into (i) the 

 anal fossa or proctod;cum, and (2) the uro-genital fossa, by means of the 

 prc-anal or perineal fold. 



The cloaca is persistent in Amphibia, Reptiles, Birds, and Monotremata; 



