MUSCULAR SYSTEM. 117 



together with the scaleni, which belong to the last-mentioned 

 group. 



The muscles of the main part of the tail retain primitive rela- 

 tions similar to those seen in Fishes : at the root of the tail, 

 however, new muscles become differentiated. 



Birds. In Birds the primitive character of the trunk-muscles 

 has disappeared far more than in Reptiles. 



This is mainly to be accounted for by the excessive develop- 

 ment of the muscles of the anterior extremity, the pectoralis 

 major more particularly, and the corresponding backward 

 extension of the breast-bone. 



External and internal oblique muscles are present, but only 

 slightly developed : this is more particularly true of the inter- 

 nal, which appears to be undergoing degeneration. No trace of 

 a trans versalis can be distinguished, but on the other hand, a 

 paired, unsegmented rectus is present. 



External and internal iritercostals are well developed, and a 

 triangularis sterni appears for the first time on the inner surface 

 of the sternal ends of the ribs. 



The dorsal portion of the trunk musculature is only slightly 

 developed in the region of the body, though very strongly marked 

 in the neck. 



All these modifications in Birds seem to be accounted for by 

 the great specialisation of the mechanisms for flight and respira- 

 tion, to assist which the greatest possible number of muscles are 

 brought into play, and which thus greatly influence the whole 

 organism : an essential difference is thus brought about between 

 Birds and Reptiles. 



Mammals. Three lateral abdominal muscles are present in 

 Mammals, an external and internal oblique and a trans- 

 versalis. Except in a single instance (Tupaia), they are entirely 

 unsegmented, and consist of broad uniform plates of muscle. To- 

 wards the middle line, they pass into strong aponeuroses, which en- 

 sheath the rectns abdominis. The latter consists of a single band 

 on each side, and possesses a varying number of myocommata; it 

 is no longer connected with the axial muscles of the neck belong- 

 ing to the same system (sternohyoid, sternothyroid, &c.), as is the 

 case in Urodeles, for the sternum is always interposed between 

 them, as it is in Sauropsida, 



In Monotremes and Marsupials, a strong pyramidalis mus- 

 cle lies on the ventral side of the rectus abdomnis. It arises from 

 the inner border of the "marsupial bones" (epipubes), stands in 

 important relation to the pouch (marsupium), and may extend 

 forwards as far as the sternum. In the higher Mammals, where 

 the epipubes are absent, the pyramidalis becomes greatly reduced 

 or entirely lost. Traces of it are, however, commonly to be met 



