284 COMPARATIVE ANATOMY 



The muscles of the foot strongly resemble those of the hand. The 

 extensor digitorum brevis, which extends across the top of the foot from 

 the heelbone to the four lateral toes, has, however, no homologue in the 

 hand. On the top of the foot also are four dorsal interosseus muscles. 



The most superficial muscle of the sole is the flexor digitorum brevis 

 connected with the four lateral toes. The quadratus plantae is inserted 

 upon the tendons of the flexor digitorum longus. The four limibricales 

 are flexors of the four lateral toes. Three muscles, abductor, flexor, and 

 adductor, connect with the great toe. The little toe also has three special 

 muscles, abductor, flexor, and opponens. The three ventral interossei 

 lie between the metatarsals. 



Pelvic Muscles. Muscles of the pelvic diaphragm, of the urogenital 

 diaphragm, and of the external genitals are, from their position, regarded 

 as pelvic. 



Those of the pelvic diaphragm are the coccygeus, which connects the 

 coccyx with the ischium, the levator ani, a muscle with three divisions 

 which lifts the anus, and the sphincter ani extemus, consisting of three 

 muscular rings surrounding the anus. All are supplied with branches of 

 sacral nerves. The levator ani complex represents the caudal flexor- 

 abductor musculature of tailed mammals, which in man, as in the anthro- 

 poid apes, with the reduction of the tail, has gained new relationships 

 with the pelvic viscera. The coccygeus represents the proximal ventral 

 caudal abductor of tailed mammals. 



The urogenital diaphragm has two muscles, a transversus perinei 

 and a sphincter urethrae. The latter closes the urethra and is under 

 voluntary control. In the male, the sphincter also compresses the prostate 

 and bulbo-urethral glands. In the female, it compresses the vagina and 

 the glands of Bartholin. 



The external genital muscles include the bulbo-cavemosus, which in 

 the female compresses the vagina and in the male increases the turgescence 

 of the penis, and the ischio-cavemosus, which serves to maintain the 

 turgescence of the penis or clitoris. 



Development of the Muscles 



The classification of muscles into striped and smooth upon the basis 

 of their finer structure, like that into voluntary and involuntary, does not 

 correspond with a classification into skeletal and visceral based upon their 

 embryonic development. 



Somatic Muscles, Derived from the Epimere. The Eye Muscles. 

 The details of the development of the six eye muscles in man are not fully 

 known. The fact, however, that they are innervated by the same somatic 

 motor nerves, III, IV, and VI, as in the lower vertebrates has led morphol- 



