1894.] MYOLOGY OF RODENTS. 259 



In Hystrice and Sphingurus only one muscle can be made out ; it 

 rises from all the cervical transverse processes and is inserted into 

 the anterior four ribs. 



In Lagostomus, Agouti, and Ccelogenys the muscle rises from all 

 the cervical transverse processes ; the fibres from the anterior three 

 or four are attached to the outer surfaces of the ribs from the 

 second to the fifth and interdigitate with the serratus magnus ; in 

 Lagostomus the sixth rib is reached. The fibres from the posterior 

 transverse processes are attached to the first rib behind the sub- 

 clavian artery. 



In the Caviidse the fibres which are attached to the side of the 

 chest come from the 3rd and 4th cervical transverse processes, 

 and are inserted into the 3rd and 4th ribs. The slip to the first 

 rib comes from all the cervical transverse processes. 



Muscles of the Anterior Extremity. 



TJie Pectoral Muscles. — As the pectoralis major and minor are 

 not always distinct muscles, I have followed Owen's example in his 

 description of Capromys l , and have divided the whole pectoral 

 mass into four different parts, which are usually easy to make out. 

 These four parts have generally the following attachments : — 



(a) The most superficial part, rising from the anterior portion 

 of the sternum aud sometimes the sternal end of the clavicle, 

 is inserted fairly low down on the humerus, often crossing 

 obliquely the fibres of the next part, which is on a deeper 

 plane. 



(,S) This portion rises from the greater part of the sternum 

 posterior to the last and is inserted into the pectoral ridge 

 of the humerus. 



(y) The abdominal portion rises from the linea alba ; being 

 closely connected to and embraced by the panniculus 

 carnosus, its fibres pass deep to ri, and are usually inserted 

 into the top of the pectoral ridge and the upper extremity 

 of the humerus. 



(S) The deep portion, which perhaps corresponds to the pector- 

 alis minor of human anatomy, rises from the cartilages of 

 some of the true ribs, close to their junction with the 

 sternum. The fibres run upwards and outwards to the 

 outer part of the clavicle, coracoid, or shoulder-capsule. 



In the Octodontidse a and /3 are almost if not completely fused, 

 y goes to the lesser tuberosity of the humerus. 2 rises from the 

 cartilages of 3rd to 6th ribs and is inserted into the outer part of 

 the clavicle and coracoid process. This arrangement applies to 

 Myopotamus, Aulacodm, and Capromys pilorides ; it also agrees 

 with Dobson's description of the muscle in Capromys melanurus '~. 



1 P. Z. S. 1832, p. 74. 



2 P. Z. S. 1884, p. 234. 



