Oy THE EPISTERXAL APPARATUS OF MAMMALS, 555 



Like those of the Hamster they are composed of hyaline cartilage, 

 which presents at some points a fibrous basis, and which also passes 

 into the periosteum without any strong line of demarcation, whilst in 

 the Hamster there is a highly developed joint between each episternal 

 piece and the clavicle. I am unable to find any such structure in the 

 Hedgehog; yet there is certainly no continuity of tissue between 

 these parts, for the apparent investment of hyuline cartilage covering 

 the sternal end of the clavicle presents elongated cells in its external 

 layers, and ultimately passes by a slightly undulating surface into a 

 tissue which more closely approximates connective tissue, and which is 

 continued into the hyaline cartilage of the episternals, in the same 

 gradual manner that it proceeds from that of the clavicle. Thus the epi- " 

 sternal piece appears to constitute a glenoid cavity for the reception 

 of the globular and articular-like end of the clavicle, and there is a 

 slight amount of mobility between the two pieces, without however 

 any true joint intervening between them. This structure corres- 

 ponds exactly with that which is transitorily observed in the process 

 of development of many joints, and in this particular presents a 

 more complete difiereutiation of parts than is observed i]i the Insec- 

 tivora above-mentioned. Amongst the Eodents, whose clavicles do not 

 reach the sternum, and whose episternals appear only as elongated 

 lateral pieces, without the median portion that occurs in Coelogenys, 

 Cavia,2iJid. Hystrix, I find, however, that occasionally, as in the rabbit, 

 a very analogous conformation is present, the so-called ligament which 

 connects the very distant end of the clavicle to the sternum agree- 

 ing completely with the above-mentioned anatomical arrangements. 

 In the wild rabbit, I find the following histological and morphological 

 construction. The rounded extremity of the clavicle, provided with 

 a calcified cartilaginous investment, becomes continuous with a liga- 

 mentous cord that diminishes in thickness as it passes towards the 

 sternum. All authors have regarded this as indifi'erent tissue in 

 those animals which possess a clavicle that does not reach to the 

 sternum. I find this cord however to be of compound character. 

 Externally it consists of a sheath composed of longitudinally ar- 

 ranged connective tissue fibres, internal to which is a network of 

 fine elastic fibres, that is itself obliquely crossed by another layer of 

 fibres surrounding a central but much smaller cartilaginous rod. 

 At its upper and thicker portion hyaline cartilage is clearly dis- 

 cernible, becoming modified as it passes towards the sternum, into a 

 peculiar soft tissue that morphologically at least is not distinguish- 



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