vin.] THE MUSCLES. 



339 



Rectus abdominis. The greatest relative size of this 

 muscle is exemplified in the class of Fishes, where it extends 

 in the mid-abdomen from the tail to the pectoral arch, and 

 thence, forwards, to the mandibular symphysis. A complica- 

 tion of division may exist, as we find in the Salamander, 

 where there is a superficial rectus lying, on each side, imme- 

 diately upon a deeper one. 



Even in man's own class this muscle may be very much 

 more developed than in him, as in the Slow Lemur, the Arma- 

 dillo, and Ornithorhynchus, where it goes to the first rib. The 

 linece transversce may be absent, as in the Hedgehog, the 

 Dolphin, and the Hyrax ; or they may be seven in number, as 

 in the Racoon ; or they may be replaced by regular abdominal 

 ribs which subdivide the rectus into a longitudinal series of 

 successive segments, as in the Chameleon. 



The pelvic origin of the muscle may be wanting, as in the 

 Cetacea, where it arises (by a long and thin aponeurosis) 

 between the dorsal and ventral muscles of the tail, from the 

 ventral side of the transverse processes of some of the mid- 

 coccygeal vertebrae. 



A slip from this muscle may be sent to the humerus, as in 

 some Armadillos and insectivorous Bats. 



Pyramidalis. This may be more largely developed than in 

 man, as in the Iguana and in Marsupials and Monotremes, 

 where it arises from the marsupial bones. It becomes enor- 

 mous in the Ornithorhynchus. 



The quadratus himborum is a nearly constant muscle ; it 

 may be distinctly developed in Reptiles, and is large in the 

 Iguana. 



The development of the intercostal muscles is of course 

 governed by the size and number of the ribs. Thus they 

 may be more or less developed than in man, or they may be 

 completely absent. 



24. The diaphragm. In the possession of a complete 

 partition, or diaphragm, man agrees with all the members 

 of his own class, but differs from all those of every other 

 class, though in some of the latter it may form an incomplete 

 partition, at the least allowing the apex of the heart to pass into 

 the abdominal cavity, as in the struthious Birds. In man's own 

 class this muscle may vary as to the extent of the central 

 tendon and as to the points of origin and insertion. Thus 

 in the Manatee (which has so many ribs) the obliquity of 

 the diaphragm is so extreme that the thorax extends back- 

 wards above the whole length of the abdominal viscera. Very 



Z 2 



