MUStTLAK SYSTEM. 121 



Into all these regions the facial nerve extended, forming divergent branches 

 and plexuses : a posterior (occipital) and four anterior (temporal, maxil- 

 lary, mandibular, and a branch to the posterior belly of the biventer muscle) 

 main branches may be distinguished. 



The complexity of the branching of the facial nerve is thus proportionate 

 to that of the facial muscles, and i.? most marked in Primates, in which the 

 musculature gradually takes on new origins corresponding to the more and 

 more highly differentiated cranial skeleton. 



The platysma myoides thus forms the matrix for the facial 

 muscles, and it represents the remnant of a musculature continued 

 forwards to the head, which has retained (e.g. in Man) an undifferentiated 

 form in the neck (Gegenbaur). 



Besides the formation of new independent muscle", modifications of certain 

 of the facial muscles also took place, which resulted in their more or less 

 complete degeneration. Thus they have become replaced by tendinous 

 aponeurotic regions (viz. the fascia temporalis, parotideo-masseterica, and the 

 galea aponeurotica of Man), or even entirely obliterated. 



Besides the platysma myoides there is a second deeper dermal system of 

 muscles of the neck, the sphincter coll i. This, like the platysma, also takes 

 on secondary relations to the head, and gives origin to the levator labii superi- 

 oris proprius, levator angxili oris, sphincter oris, depressor tarsi, buccinatorius, 

 and the proper muscles of the nose. The facial muscles not mentioned here 

 arise from the system of the platysma. 



MUSCLES OF THE APPENDAGES. 



The following important factors must be taken into consi- 

 deration with regard to the muscles of the appendages : (1) the 

 homologies of the parts of the skeleton ; (2) the relative positions 

 of the neighbouring soft parts ; and (3) the nerve-supply. 



The most primitive condition of the muscles of the extremities 

 is met with in Dipnoi, more particularly in Ceratodus. In this 

 case, the musculature of each surface of the fin forms a uniform 

 mass, there being hardly any indication of a separation into definite 

 layers and groups. Everything goes to prove that all the muscles 

 of the appendages are to be looked upon as derivatives 

 of the lateral muscles of the trunk. 



Two principal groups of appendicular muscles may always 

 be distinguished; one lying in the region of the pectoral and 

 pelvic arches, the other in the free extremity. 



In the tins of Fishes, very simple conditions of the muscles 

 are met with ; in Amphibia, on the other hand, in correspondence 

 with the more highly-differentiated organs of locomotion, con- 

 siderable complication is seen, and there is a much more marked 

 separation into individual muscles, corresponding with the different 

 sections of the extremity. In Fishes, only simple elevators, 

 depressors, and adductors, for moving the extremity as a 

 whole, are present, while from Amphibia onwards there are added 

 rotators, flexors, extensors, and adductors of the upper arm 

 and thigh, of the fore-arm and shank, and of the hand and foot. 

 The digits are also moved by a highly-differentiated musculature. 



