THE MUSCULAR SYSTEM 253 



in the stcrnalis muscle, an element of very rare occurrence and 

 proven to belong here by its occasional relationship with both 

 pectoralis abdominis and the elements of the axillary arch, 

 as well as by its innervation from the anterior thoracic nerve, 

 in common with the foregoing (Fig. 69). The st emails 

 muscle has been recently shown to occur much more frequently 

 in the Japanese than in Europeans (13 per cent, against 4 

 per cent. ) . It lies superficial to the pectoralis major, and when 

 well developed may be so contracted as to be plainly visible 

 from the exterior. 



Another system of integumental muscles is derived from the 

 visceral musculature and appears in its simplest form in the 

 sphincter colll of amphibians, reptiles and birds, and is itself 

 a direct descendant of a selachian muscle, the superficial con- 

 strictor. The fibers of this sheet enwrap the neck region and 

 in turtles and birds the muscle is well developed and covers the 

 entire neck. In mammals this sheet differentiates into two 

 layers, a more extensive superficial layer, the platysma, 

 and a smaller and deeper layer, which retains the original 

 name of sphincter colll. The fibers of these two sheets run 

 primarily at right angles to one another, those of the platysma 

 being directed upwards and towards both snout and ear, those 

 of the sphincter in more nearly the original direction across 

 and around the neck. 



In following the phylogenetic series through marsupials 

 and lemurs to primates, a considerable extension of both of 

 these layers over the face and head is noticed, and as they 

 meet the eyes, nose, ears, and lips there is seen a pronounced 

 'tendency to form special slips for the regulation of these parts t 

 a tendency precisely similar to that of the ventral panniculus 

 in the case of the marsupial pouch and the cloaca of the mono- 

 tremes. There is thus formed the extensive system of facial 

 muscles, often termed the "mimetic" muscles, which become 

 so highly differentiated in the apes and in man, and this grad- 

 ual differentiation can be clearly followed in the phylogenetic 

 series (Fig. 70). 



The superficial sheet or platysma extends upwards across 



