130 On the Muscular Anatomy of the Marsupials, 



It is evident from the foregoing that the marsupial bones, 

 supporting as they do occasionally a considerable weight, are 

 held in equilibrium by the opposing action of two levators and 

 three depressors ; so that, in active motions of the mother, the 

 bone vibrates, balancing itself backwards and forwards as it is 

 impelled by the inertia of the guts and of the young in the pouch, 

 and by the opposing actions of the muscles, and thus perpetually 

 '' tips '' the cremaster muscle that winds round it, exciting it to 

 contract upon the mammary gland, and so feed the helpless 

 young one suspended, by an almost organic union, from the 

 nipple of the gland inside the pouch. 



By means of this self-acting mechanism the young is per- 

 petually fed, and the mother marsupial relieved from a care that 

 would infallibly prove too great for her little modicum of brains. 



I may mention that I have confirmed the foregoing explana- 

 tion by the careful dissection of the male Marsupials, in which 

 I find that the cremaster muscle passes from the abdominal ring 

 to the testis in such a manner as to clear the marsupial bone ; 

 so that the cremaster in the male is not tipped by the marsupial 

 bone at every motion of the body, as it must be in the case of 

 the female marsupial, carrying her young permanently in her 

 pouch. 



II. The Quadratus femoris Muscle in the Marsupials. 



This seems to me to be one of the most remarkable muscles 

 found in the Kangaroos, and is intended to assist in the support 

 of these animals while resting in their favourite attitude, sitting 

 on the tail and hind legs. It takes its origin from a large tri- 

 angular surface on the ischium, and thence converging in a 

 pyramidal mass to a point, is inserted into a special trochanter 

 or tubercle developed for its reception in the middle of the 

 posterior surface of the shaft of the femur. Fully two-thirds 

 of this pyramidal mass are composed of tendinous fibres, the 

 remaining third being muscular; so that the whole may be re- 

 garded as forming an elastic tendon. When the animal sits 

 upon its tail and legs in the manner above described, it some- 

 times places its feet so far forward, and its tail so far back, that 

 a mechanical observer is at once struck by the apparent want of 

 strength in the arch on which the weight of the body is sup- 

 ported, and feels disposed to come to the conclusion that the 

 act of sitting on its tail must be a fatiguing one to the kangaroo. 

 Observation of their habits, however, abundantly proves the 

 contrary, and shows that the animal prefers this attitude to any 

 other. 



The explanation of this difficulty is to be found in the semi- 

 tendinous condition as well as great size of the quadratus femoris, 



