55S THE jS^ATUKAL HISTOEY EE VIEW. 



the sternum into that of the clavicle, and consequently surrounded 

 the fissure which existed between the clavicular surface of the cartilage 

 and the clavicle, thus completing the articular cavity. The two 

 cartilaginous episternal pieces, possessed a certain amount of elasticity. 

 They were closely approximated to one another at the anterior border 

 of the manubrium sterni, and the outer space was occupied by a short 

 lancet-shaped cartilaginous prolongation (m), the minute structure 

 of which was the same as that of the larger pieces of cartilage. From 

 the above description, it follows that these cartilaginous pieces are 

 not to be considered as detached portions either of the clavicle or of 

 the sternum. Erom the clavicles they are separated by a well- 

 defined line, and a well-marked histological difference ; and at one 

 part the separation is so perfect that a kind of joint is formed : whilst 

 they are also difterentiated from the sternum by their microscopical 

 structure with equal clearness, so that they can neither be regarded 

 as simple processes of the sternum, nor as sternal prolongations of 

 the clavicles. They consequently exactly agree with the above- 

 mentioned intermediary pieces between clavicle and sternum ; and 

 if they are in those instances to be termed episternals, the same 

 nomenclature must also be applied to these cartilaginous structures 

 of the mole. The smaller degree of independency resulting from 

 their more intimate connection with the sternum, constitutes no 

 serious objection to this view : for we have already seen that in the 

 Hedgehog the episternal cartilage is very intimately connected with 

 the clavicle ; and that on the other hand, in Didelphys the T-formed 

 cartilage is more closely connected with the sternum than with the 

 clavicle. 



At a much lower grade of development, yet still by the light of 

 the structures just described in the mole, readily recognizable as 

 belonging to the same series, are the homologous parts in the Squirrel. 

 If in this animal, the sterno-clavicular articulation be examined from 

 the anterior surface of the body (Fig. 9), the expanded sternal ex- 



FiG. 9. 



C 



Episternal apparatus of Sciurus vulgaris (Squirrel), seen from the outer side 

 c, clavicle ; c', first rib. 



