84 



THE OSTEOLOGY AND MYOLOGY 



They probably also tend to preserve the shape of the pouch, acting to this eflect like the 

 ribs of the thorax, though in far less degree. But their most important office is believed to 

 be the increasing of the power or effect of the female cremaster, or " ilio-marsupialis," 

 which muscle winds around them as aro.und pulleys on its way from the haunch-bone to the 

 mammary glands. Still this is only an assumption ; for the strong suctorial power that 

 the youngest animals have been found to possess, renders it improbable that voluntary mus- 

 cular effort in compressing the glands on the part of the mother is called for. It seems 

 highly improbable that they play any part in the act of parturition, as some have sup- 

 posed. Owen regards them as homologically the hnemapophyses of the last lumbar verte- 

 brae ; and as teleologically belonging " to the category of tlie trochlear ossicles, commonly 

 called sesamoids," being " developed in the tendon of the external oblique which forms the 

 mesial pillar of the abdominal ring, as the patella is developed in the tendon of the rectus 

 femoris.'" 



Femur. — (Fig. 25.) The thigh-bone is long — over three inches in length — perfectly 

 straight, stout, and nearly cylindrical. There is no decided " linea aspera," the posterior 



surface of the shaft being, instead, somewhat flattened, and pre- 

 senting at the junction of its middle and lower thirds a tuberosity 

 — almost a " process," which is rough for attachment of the adduc- 

 tor magnus. The shaft is slenderest at its middle, whence it 

 gradually enlarges to the condyles. These have a backward 

 extension ; anteriorly, the mesial groove for the rectus tendon is 

 broad and very shallow^ ; posteriorly, a very deep notch separates 

 the two articular surfaces. The condyles are of nearly the same 

 size ; the inner one is the longest, and has the most backward and 

 inward production ; the outer is shorter and broader, and with a 

 sharper external border. The lateral aspects of both are promi- 

 nent, and rough for ligamentous attachments. The inner articular 

 surface is narrow, and transversely flat or slightly convex ; the 

 outer is broader, and a little concave in the same direction. The 

 head of the bone is far to one side of the axis of the shaft, held 

 obliquely upward and inward at an angle of 45° upon a short, 

 stout neck. A transverse section of the head and neck together 

 would be in the same plane as a similar section across the two 

 condyles — there being no twisting of the head either forward or 

 backward. The articular surface is almost a hemisphere ; the well- 

 marked depression for the lifjamentum teres is situated near the inner margin of the hem- 

 isphere. The constriction of the neck is slight. A slight ridge upon the postero-internal 

 aspect of the neck soon rises into the sharp, well-marked, triangular trochanter minor, and 

 suddenly subsides. This trochanter is situated upon the back and inner aspect of the shaft, 

 nearly in the same plane as the latter. A stout ridge crosses the neck behind, running 



Finr. 25.— Rijxht Femur. 



Hn consequence of (or causinf^?) the imperfection of the 

 patella. According to Owen, " the intermediate anterior 

 groove for the patella is well marked in the Perameles, 

 where the patella is fully developed, but is broad and very 

 ehallow in the Plialanf;ers and Dasyures, where the tendon 

 of the rectus muscle is merely thickened, or offers a few 



irregular specks of ossification ; and the corresponding sur- 

 face on the Wombat and Koala is almost plane from side to 

 side ; in these marsupials and in the Myrmccobius the 

 patella is wanting." The opossum is another instance of 

 correspondence between this groove and the patella; 

 exemplifying a law that perhaps has no exceptions. 



