122 COMPARATIVE ANATOMY. 



In cases where, as in Primates, the anterior extremity is con- 

 verted into a prehensile organ, new groups of muscles appear, 

 known as 'pronators and supinators. The former are derived 

 from flexors, the latter from extensors. 



On account of the relatively small amount of movement of 

 the pelvic arch as compared with the pectoral arch, one would 

 naturally not expect similar groups of muscles connected with 

 these two regions : entirely different relations are here frequently 

 to be met with. 



DIAPHEAGM. 



The first traces of a muscular partition-wall between the 

 thoracic and abdominal cavities are to be met with in Urodeles. 

 In them we find circular and semicircular fibres of the transversalis 

 muscle passing inwards between the pericardium and the liver. In 

 Chelonians, and more particularly in Crocodiles and Birds, where 

 the muscular fibres concerned in the formation of the partition 

 arise from the ribs, 1 the indications of a diaphragm are much 

 plainer, but there is not a complete separation into thoracic and 

 abdominal cavities. A complete dome-shaped diaphragm, arising 

 from the vertebral column, ribs, and sternum, appears first in 

 Mammals, and is of great importance in respiration, as it allows 

 of a lengthening of the thoracic cavity in a longitudinal direc- 

 tion. It is supplied by the phrenic nerve, which arises from one 

 (4th to 6th) of the cervical nerves, and usually consists of two 

 parts, a pericardial and a pleural, arising independently of one 

 another. The former is fibrous, and forms the central tendon, 

 while the latter is muscular. In some cases (e.g. Echidna, Pho- 

 csena) the diaphragm is entirely muscular. The Mammalian dia- 

 phragm is probably not the homologue of the so-called diaphragm 

 ( of other Vertebrates. 



BIBLIOGRAPHY. 



BAIIDELEBEN, C. Muskeln und Fascie. Jenaische Zeitschr. Bd. XV. N.F. VIII. 



CUNNINGHAM, D. J. The Relation of Nerve-Supply to Muscle- Homology. Journ. of 

 Anat. and Physiol. Vol. XVI. 1882. Report on some Points in the Anatomy of 

 the Thylacine, &c., with an Account of the Comparative Anatomy of the Intrinsic 

 Muscles of the Mammalian Pes. " Challenger " Reports, Vol. V. The Structure 

 and Development of the Suspensory Ligament of the Fetlock in the Horse. American 

 Naturalist, Vol. XIX. p. 127. 



DOBSON, G. E. On the Digastric Muscle. Trans. Linn. Soc. Vol. II. Part 5. 

 Monograph on the Insectivora, 1882. Homologies of the Long Flexor Muscles. 

 Journ. of Anat. and Physiol. Vol. XVJI. 1883. On the Rectus Abdominis and 

 Sternalis. Ibid. The Comparative Variability of Bones and Muscles. Ibid. 1885. 



DUGES, A. Rech. sur V osteologie et la myologie des batracicns a leurs differents ages. 

 Paris, 1834. 



1 In Birds, two entirely different structures have been described as a diaphragm. 

 Comp. the chapter on the air-sacs of Birds. ) 



