ON THE EPISTERNAL APPARATUS OF MAMMALS, 551 



viduals. In older animals, however, calcification of the cartilage 

 takes place quite regularly at one or other of the two ends of each 

 cartilage, as is represented in Fig. 3. In other species (3fics syU 

 vaticus, minutus) the episternal structui'es present nearly the same 

 general features. In 3Ius sylvaticus the two external ends of the 

 episternals are in close approximation to one another. In IIus mi- 

 nutus the cartilaginous episternals appear at first sight to be directly 

 continuous with the cartilaginous extremities of the claA'icles. At 

 the place of transition from one to the other, however, the cartilage 

 alters in structure, and is found to contain long fusiform and trans- 

 versely-arranged cells ; and it is even possible in some instances, to 

 detect a cavity in this tissue forming a true joint, though certainly 

 one of the very lowest grade of development. 



The two episternal pieces are club-shaped, the blunt end being 

 directed towards the sternum, and the pointed end towards the 

 clavicle. The latter extremity terminates in a cup-shaped expansion 

 covered with hyaline cartilage which receives the clavicle. The va- 

 riations presented by other species are insignificant. K the slight 

 constriction, which is present in the Mus musculus, be conceived to 

 be somewhat greater, the clavicular extremity of the episternum would 

 exhibit the same cup-sbaped process that is visible in 3Ius minutus. 

 The sternal ends of the episternals are as far removed from one 

 another as in the house-mouse. In their interior are two osseous 

 nuclei— a posterior one of roundish form, and another immediately 

 in front of it resembling a wedge. 



The episternals of the Eat {Mus decumanus) are somewhat difierent 

 from those of the smaller Muridae. On a superficial examination 

 they appear to be very solid and moveable pieces about y ^-^h to -oVth 

 of an inch in length, lying between the clavicle and the sternum, and 

 connected with both those bones by strong fibrous tissue. It is im- 

 possible to discover any articular cavity at either of their extremities. 

 The dense episternals can be easily ascertained to be composed of 

 bone ; for on making a vertical section, the sternal end of the bony 

 clavicle is seen to be invested with a layer of hyaline cartilage, and 

 to contain numerous medullary cavities that are wide internally, and 

 radiate tow^ards the surface. Inmiediately continuous wdth this is a 

 layer of fibrous tissue, which again passes into the hyaline cartilagi- 

 nous investment of the cancellated bony mass, that constitutes the 

 greater part of the episternal piece. Inferiorly the ligamentous 

 bands do not pass on to the episternal bone itself, but are continued 



