184 COMPARATIVE ANATOMY 



of external and internal intercostals. 1 The subvertebral muscle is 

 represented by a lonyus colli and recli capitis antici. What 

 has been said above with regard to the quadratus lumborum 

 and to the differentiation of the dorsal portion of the trunk- 

 muscles in Reptiles applies essentially also to Mammals, in which 

 also the metamerism of the dorsal body- wall is retained longer 

 than that of the ventral. 



In the caudal musculature, flexors, extensors, and abductors 

 may be distinguished, and their degree of development is pro- 

 portional to that of the tail : in Man, for example, they become 

 reduced, and some of them (the pubo- and ilio-coccygeus) have 

 undergone a change of function, giving rise to the levator ani or 

 " pelvic diaphragm," consisting morphologically and phylogenetic- 

 ally of three portions (pubic, ischiatic, and iliac). 2 



B. Muscles of the Diaphragm. 



The formation of a diaphraym results from a gradual sub- 

 division of the crelome (pleuroperitoneal cavity) into pleuro- 

 pericardial and abdominal portions, and the differentiation of the 

 serous membranes which line these (phur a, pericardium, peritoneum} 

 can only be understood in connection with the complicated 

 development of the primitive urinogenital folds, liver, lungs, and 

 great veins, and so cannot be dealt with in this place. 



From the Sauropsida onwards, a more or less distinct 

 separation of the pleural and peritoneal cavities is seen. In 

 Chelonians and Lizards a partition is present between these 

 chambers, but this is complete only in Crocodiles and Birds. 

 Subperitoneal muscular elements are present which connect it 

 with the vertebral column and ribs, but the innervation of these 

 precludes any homology with the diaphragmatic muscles of 

 Mammals. 3 It is here therefore only a case of analogy ; and it must 

 be remembered that in the Sauropsida the pericardium lies in the 

 general peritoneal cavity. 



1 The very variable serrati postici superior and inferior are peculiar to 

 Mammals above Monotremes. They do not form a single layer, but are indepen- 

 dent of one another, and are derived respectively from the external and internal 

 intercostals. 



2 It is doubtful how far the external sphincter of the anus, the muscles in 

 connection with the external generative organs, and the tramversm perinei pro- 

 fundus are derivable from the original sphincter cloacae of the Amphibia and 

 Sauropsida. In Mammals the pubo-coccygeus (or the pubic portion of the levator 

 ani), as well as the sphincter ani externus and bulbo- and ischio-caverno-n, are 

 considered to represent separate portions of the integumentary muscle which 

 primarily extended over the greater part of the trunk. 



3 Amongst the Amphibia (Rana) fibres from the trans versus which extend on 

 to the gullet have been compared to a diaphragm, but the relations are here 

 quite different to those of the muscles of the mammalian diaphragm, in the forma- 

 tion of which the rectus abdominis plays an important part. In Birds, two 

 entirely different structures have been described as diaphragms (cf. under 

 Air-sacs). 



