A Contribution to tlic Aimtoniy of Siren lacertina. 659 



This muscle is best prepared by separating the t^Yo portions from 

 above downward, as deeply as possible without cutting. 



Then the anterior portion can be cut away from its origin and 

 turned back. This will bring into view the two branches of the facialis 

 above mentioned, close to the skull and beneath the projecting ridge 

 of the tympanic bone. Branches will be seen running from Ramus ju- 

 gularis facialis and supplying both portions of the muscle. The posterior 

 portion may now be removed from its origin in like manner, and the 

 whole muscle drawn away and cut oif near its insertion in the man- 

 dible. After the removal of this muscle, the connecting nerve between 

 facialis and the vagus group will be seen close to the walls of the 

 skull, and immediately above the upper end of the cerato-hyal. 



Origin. Pars anterior. This part arises from Os petrosum 

 somewhat behind and below the lateral ridge of Os tympanicum, also 

 from the external surface of the fascia between the skull and the 

 mandible. Pars posterior. This portion is much larger and more 

 convex than the former and fills the space between the cerato-hyal 

 and the suspensorial apparatus of the mandible. It is entirely covered 

 by the digastric fascia but no fibres arise from it. This part takes 

 its origin from the external surface of the cerato-hyal, and by a few 

 fibres from the membrane between the cerato-hyal and the first 

 gill-arch. 



Insertion. The posterior portion forms for its insertion a flat 

 tendon and into this the fibres of the anterior portion are inserted, 

 so that finally both portions find insertion by a common tendon into 

 the posterior end of the mandible behind the articulation. 



Innervation. As before described, from Ramus jugularis 

 facialis. 



Conclusion. With regard to the action of this muscle, it is 

 clearly the principal muscle used in opening the mouth, since from 

 its insertion behind the articulation, it cannot be used as a muscle 

 of mastication. In addition to this use, it functions undoubtedly as 

 a muscle of aspiration. „Beim Luftschnappen der Perennibranchiaten 

 dagegen wird nach Weise der Fische das Maul geöffnet und geschlossen. 

 Dasselbe mag beim Wasserathmen der Fall sein. Hierbei spielt, wie 

 mir scheint, die hintere Partie des Digastricus eine wichtige Rolle. 

 In der That gibt sie sich durch Oefftiung der Kiemenspalte als ein 

 zur Kiemenathmung bestimmter Muskel zu erkennen" (J. 53, 54). 



Concerning its morphological value, only the anterior portion can 

 represent the human digastricus and of that, only the posterior belly, 



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