ME, G. E. DOBSON ON THE DIGASTEIC MUSCLE. 263 



almost in the same right line, deglutition is easily effected hy the mylo- and genio- 

 hyoid muscles drawing the hyoid hone and larynx forwards and upwards, so as to allow 

 the masticated mass to get hehind them, and so hring it within the grasp of the pharyn- 

 geal muscles ; hut in those animals which feed while in the erect or semi-erect position 

 and the head is hent forwards, so that the cavity of the mouth is at right angles with 

 the oesophagus, it is evident that deglutition must be a much more complex action. In 

 that position the mylo- and genio-hyoid muscles are relaxed, and cannot act efficiently 

 in drawing the hyoid hone upwards and forwards, so as to allow the masticated mass to 

 pass into the oesophagus, into which it has to pass, in fact, round an angle. The difficulty 

 is got over by the connection of the digastric with the hyoid bone. This muscle during 

 the act of deglutition causes the hyoid bone, larynx, and base of the tongue to move 

 through a segment of a circle, the anterior part of the muscle drawing these parts 

 forwards ; they are then elevated by the joint action of the anterior and posterior bellies, 

 and finally drawn backwards by the posterior bellies, so as to force the masticated mass 

 into the oesophagus. 



This action of the digastric is well known, but its relation to the position of the head 

 while the animal is feeding appears not to have been generally recognized. 



With respect to the great development and united condition of the anterior bellies of 

 the digastrics so general in Quadrumana, and not uncommon in Podentia, it appears to 

 me to be related to the inefficiency of the mylo- and genio-hyoid muscles in the habitually 

 bent forward state of the head. Their separate condition in man is probably due to 

 shortening of the rami of the mandible and greater depth of the symphysis menti, which 

 bring the mylo- and genio-hyoid muscles into a more favourable position for action, as 

 well as affording more extensive attachments. Nevertheless united digastrics have been 

 noted as a muscular abnormality in a few cases, evidently recurrences of a once normal 

 condition of these muscles in ancestors provided with much longer jaws than we have ; 

 and I suspect that, when the muscular anatomy of the prognathous inhabitants of Africa 

 has been more fully examined, united digastrics will be regarded as a much less uncommon 

 human muscular anomaly than it uow is. 



EXPLANATION OF PLATE XXV. 



Fig. 1. Myoxus arellanarius. United digastric muscles (enlarged). 



Fig. 2. Mus natalensis. Dissection of inferior gular region, showing the position and form of the 



united digastrics. 

 Fig. 3. Tupaia ettioti. Dissection similar to the preceding. 

 Fig. 4. Ditto. Digastric muscle, showing the oblique tendinous intersection passing across from the 



infero-esternal margin of the posterior belly of each moiety to support the posterior free margin 



of their united anterior bellies, uniting with the corresponding tendinous band from the 



other side (enlarged) . 

 SECOND SERIES. — ZOOLOGY, VOL. II. 37 



