208 



*ANIMAL MECHANICS. 



power of showing his feelings by the play of his lips, But also 

 confers upon him the dishonest advantage of concealing their 

 expression by means of a barricade of red, black, or grey 

 bristles. 



The sphincter muscles already described — viz., of the iris, 

 eye, and mouth, have a corresponding set of antagonist radi- 

 ating muscles, intended to open the apertures round which 

 the sphincters are placed ; but there are other sphincters sur- 

 rounding apertures, which it is not necessary to open by 

 muscular agency. Such sphincters are the anal and vaginal 

 sphincters, which close their respective apertures, and are 

 not interfered with by any antagonist muscles. 



The anal sphincter consists of two distinct parts ; an inter- 

 nal part, composed of circular fibres of involuntary muscle, 

 and an external part, composed of elliptical fibres, which are 

 under the control of the will. The fibres of the elliptical 

 sphincter take origin from behind, in the os coccygis, and pass- 

 ing round the anus, unite in front of it, at the central point, 

 (tendoperincei)', from this point again, in the female, the fibres 

 of the vaginal sphincter proceed, opening out to surround the 

 vaginal orifice and vestibule, and are finally inserted on the 

 corpora cavernosa clitoridis. The two sphincters thus form a 

 muscular sheet, resembling the figure 8, and may act together 

 or separately. 



The levator ani, which forms the pelvic diaphragm, does 

 not act as an antagonist to the anal sphincter; on the contrary 

 its action aids the sphincter, for those of its fibres which blend 

 with the sphincter fibres are tangential to the curve of the 

 latter (like the fibres of the canine muscle in the mouth), and 

 therefore assist instead of opposing the constricting action of 

 the sphincter muscle. 



During parturition, the levator ani and sphincter ani co- 

 operate to close the aperture of the anus, so as to allow the 



