40 THE INVOL UNTAR Y NER VO US S YSTEM 



all the muscles surrounding the Wolffian and Mlil lerian ducts, 

 and since these ducts arise from the segmental duct, it may be 

 called the segmental duct system of involuntary muscles (Fig. 8). 



Before giving the evidence in support of this assertion I will 

 first recall to the mind of my reader the embryological evidence 

 of the meaning and fate of the segmental duct, so that the nature 

 of the musculature in question may be clearly understood. 



The primitive organs of excretion were the pronephric organs, 

 which excreted into the cloaca by a single duct on each side 

 known as the segmental duct. Later on, in the fishes and am- 

 phibia, another set of segmental tubules was formed, in close re- 

 lationship with the generative organs, which excreted also into 

 the segmental duct and so into the cloaca. These were the 

 mesonephric organs or Wolffian body, as they are called. The 

 neighbouring generative organs also come to make use of the 

 segmental duct, and the original duct becomes split into two 

 ducts, the one the Mullerian, conveying the genital products, the 

 other the Wolffian, conveying the excretory products. Later on 

 still, in reptiles, birds and mammals, more kidney tubules are 

 formed in segments posterior to the mesonephric, known by the 

 name of the metanephric tubules, and these form the permanent 

 kidney. The Wolffian body now takes no part in urinary excre- 

 tion, only the generative gland remaining, whose duct is the 

 Mullerian duct. 



The formation of the duct of the permanent kidney is thus 

 described by Keith. " It arises as a bud from the dorsal side 

 of the Wolffian duct, near the termination of that duct in the 

 cloaca. At first it is a stalked bud with a narrow lumen ; it 

 rapidly extends forwards to the lumbar region behind the Wolffian 

 body and behind the peritoneum. The stalk of the bud forms 

 the ureter. The connexion of the stalk with the Wolffian duct is 

 lost ; the termination of the ureter migrates along the duct till it 

 reaches that part of the cloaca which afterwards forms the 

 bladder." 



The Mullerian ducts become the Fallopian tubes and by their 

 fusion the uterus is formed. The terminal part of the Wolffian 

 ducts in the female disappear : in cases where they persist they 

 are known as the ducts of Gartner. In the male, the vas de- 

 ferens and ejaculatory duct are formed from the Wolffian duct, 

 and enter into the uro-genital sinus at the place where the sinus 



