MYOLOGY OF THE ORNITHORHYNCHUS. 137 



(a). A small muscle somewhat resembling another mylo-hyoid, 

 arises with its fellow from the hyoid on the median line, and proceeds 

 forward and outward, spreading over the jaw at its narrowest part, 

 to be inserted into the lower lip. 



Another little muscle lies upon the outside of the ramus of the lower 

 jaw, taking definite origin, fleshy, from the fossa at the end of the 

 groove in the bone ; it is distributed to the integument of the lip. 

 Transverse fibres of the panniculus, and the muscle a, also contribute 

 to the. fleshiness of the part. 



Other muscles of this region could not be dissected owing to the 

 condition of the specimen. 



IV. THORACIC MUSCLES proper. 

 These were not dissected. 



V. ABDOMINAL MUSCLES. 



The muscular walls of the belly conform to the usual marsupial type 

 in the great size and fleshiness of the obliquus externus, extreme thin- 

 ness and verticality of the obliquus interhus, fleshiness of transver- 

 salis, thoracic prolongation of rectus, and extent of pyramidalis. 

 They are chiefly noticeable for the absence of inguinal opening. The 

 quadratus lumborum, often referred here, has been already considered. 



Obliquus abdominis externus. Arises by fleshy digitations from all 

 the vertebral ribs except the two first the first digitation being con- 

 tinuous with the lower slip from the serratus magnus ; and separation 

 into digitations becoming obsolete on the last 2-3 ribs ; with origin 

 also from lumbar fascia and tip of the ilium. The posterior border of 

 the muscle presents a concavity towards the spine, the most distant 

 point being at the middle of the thorax, about 3 inches from the 

 vertebrae. Except at the extreme lower portion the muscle is fleshy, 

 with the usal direction of its fibres ; these continue as far forward as 

 the outer border of the rectus. The insertion is aponeurotic into the 

 whole linea alba, and symphysis pubis ; and fleshy into whole length of 

 marsupial bone, its upper edge ; at the tip of the latter, there is also 

 an aggregation of fleshy fibres of insertion. There is no arrangement 

 for abdominal rings or 'pillars.' 



Obliquus internus. Extremely thin, and with some difficulty de- 

 monstrable from the transversalis, even in its muscular portions ; but 

 on holding the wall up to the light, the fibres may be distinctly seen 

 decussating with those of the transversalis nearly at right angles. 

 The aponeuroses of the two are blended and completely inseparable 

 after passing beyond the tolerably well defined 'linea semilunaris,' 

 where the muscular fibres of both end. The fleshy part arises mostly, 

 if not wholly, from the iliac extremity, and passes upward with little, 



COMMUNICATIONS ESSEX INSTITUTE, VOL. VI. 18 March, 1871. 



