MYOLOGY OF THE ORNITHORHYNCHUS . 143 



edge of the thorax, and there, near the median line, a slight cellular 

 interval may occur between thoracic and abdominal portions. The 

 chest portion is of nearly uniform, and great thickness ; there is no 

 evident distinction of a deep-seated from a superficial part ; but the 

 outer half of the episternal portion and the acromial portion are to- 

 gether * separable from the sternal portion, by a slight cellular interval 

 along a line representing the posterior border of the muscle below 

 described as the anterior part of the deltoid. (This last named muscle 

 is crossed at right angles, overlaid, and mostly hidden by the pec- 

 toralis.) The rather thin outer border of the pectoralis is nearly in a 

 straight line from the symphysis pubis to the shoulder ; the thoracic 

 part of this border lies nearly parallel with the anterior border of the 

 obliquus abdominis. The thick convex anterior border dips down 

 over the posterior border of the deltoid to the humerus. All the fibres 

 of this great muscle converge without twisting to an extensive linear 

 insertion by a short stout tendon into the pectoral ridge of the hu- 

 merus. The acromio-episternal bundle of fibres is set in much lower 

 down than the others, from which they are virtually separated by the 

 insertion of the slip from the panniculus. The pectoralis has the 

 usual action, carried to a high degree; it is also, owing to the great 

 development of the pectoral crest, an unusually powerful rotator of 

 the humerus, an action in which the latissimus assists. The purpose 

 here subserved is evident on reflection upon the way the paddle-like 

 hands should strike the water when the whole arm is forcibly extended 

 in giving the backward stroke. 



(b.' From the scapular arch.) 



Deltoid. (According to high authority, the deltoid is double, and 

 the two muscles about to be described may constitute its two halves ; 

 but the anterior of these, which is overlaid and covered by the pec- 

 toralis, would hardly recall a deltoid by any physical feature.) The 

 posterior part is of pyramidal shape, with thick fleshy origin from 

 the most anterior (highest) part of the scapula, and a low insertion 

 on the humeral crest by a rather long tendon. Some of its fibres of 

 origin appear almost continuous with those of the posterior trape- 

 zius ; while it is almost blended with the pectoralis at its insertion. 

 The anterior portion lies upon the epicoracoid plate, conforming 



* These portions together are in the ordinary position, and have much the 

 appearance, of a deltoid in fact, they resemble one much more than the muscle, 

 below described as " anterior deltoid," does. We are in doubt of the accuracy of our 

 identification of the muscle we describe as anterior portion of the deltoid, but can 

 come to no more satisfactory conclusion, without identification of what is described 

 in a preceding paragraph as " Subclavius ? Pectoralis minor?" a determination 

 that we cannot at present make. One of the inner pectorals of birds may furnish 

 the clue. 



COMMUNICATIONS ESSEX INSTITUTE, VOL. VI. 19 April, 1871. 



