| OCR Text |
Show 1896.] MYOLOGY OF RODENTS. 167 /3 rises from the whole length of the sternum ; y (the abdominal fibres) are feebly marked; o (the part corresponding to the pec-toralis minor) comes from the lower true rib-cartilages and runs forward and outw7ard to the head of the humerus and to the cora-coid process. Heteromys only differs from this description in having the abdominal fibres more closely united with the panni-culus than in any other animal. In the G-erbille, a and /3 are fused, while y and W unite near their insertion, which is into the capsule of the shoulder and into the humerus just internal to the pectoral ridge. The Hamster differs from the last only in having the abdominal fibres better developed and running quite separately to the cora-coid process. The Vole closely resembles the Gerbille, but is remarkable for the great development of the fibres from the cartilages (o) ; these fibres unite with y, and are inserted into the coracoid. In the Rat and Mouse the arrangement is almost identical with that of the Gerbille. In Siphneus Milne-Edwards describes a large and distinct pec-toralis minor inserted into the coracoid process and coming from the second rib ; this is interesting when one notices the similar arrangement in the Vole, an animal to which he regards Siphneus as being more closely related than to the Mole-Rats x. Rhizomys resembles Cricetomys except that h is larger, rises from the 2nd to the 7th costal cartilages, and is inserted into the clavicle as well as the head of the humerus. Georychus has a and j3 fused ; y is inserted into the neck of the humerus ; <$ rises from the 2nd, 3rd, and 4th costal cartilages and is inserted into the coracoid and shoulder-joint. In Bathyergus the pectoral mass is very large, and resembles the last in having a and j3 fused ; but near the insertion the posterior fibres become tucked under the anterior and are inserted deep to them into the pectoral ridge. The abdominal fibres (y) rise from the end of the sternum to the middle of the linea alba ; they are inserted together with a large part of the latissimus dorsi into the capsule of the shoulder. The pectoralis minor (S) rises from the posterior two-thirds of the sternum and the posterior true rib-cartilages, and is inserted into the outer half of the clavicle, having its anterior border in contact with the posterior border of the subclavius; between these two contiguous borders the external anterior thoracic nerve emerges to reach the superficial parts of the muscle. Sterno-scapularis.-In the greater number of the Myomorpha this muscle is only represented, as in Man, by the subclavius, which rises from the cartilage of the first rib and is inserted into the outer half of the posterior border of the clavicle. Georychus and Bathyergus are the only exceptions which I have met with ; in these there is a large and distinct claviculo-scapularis, as in the Hystrico-morpha. In Siphneus, according to Milne-Edwards, the arrangement is the same. 1 ' Eeckercbes des Mammiferes,' tome i. p. 99. |
