496 COMPARATIVE ANATOMY. 



differentiated, inasmuch as the muscles between the ribs, or their 

 equivalents, are still portions of the lateral muscles. The ribs them- 

 selves lie in the intermuscular ligaments. In the higher divisions 

 of the Vertebrata a more distinct differentiation obtains. These 

 muscles are best developed in the Ophidii. The muscles which are 

 found between the rudimentary ribs, which are fused with the 

 vertebras, or between the transverse processes (intertransversarii), 

 also belong to this group of intercostal muscles. So, too, do the 

 levatores costarum, and the muscles which lie on the inner surface 

 of the wall of the thorax (thoracici interni), and the scaleni. The 

 size of all these muscles varies very greatly according to the extent 

 and power of movement of the ribs ; special retractors may be added 

 to the elevators of the ribs, as in the Ophidii. 



The broad ventral muscles must also, in all probability, be 

 regarded as belonging to the system of intercostal muscles ; these 

 are found in those regions of the ventral wall where there are no 

 ribs. They consist of the obliquus externus, obliquus interims, 

 and transversus abdominis. The obliquus externus corresponds to 

 the intercost. externus, and the interims to the intercost. internus. 

 The tendinous bands found in many Amphibia, and in the Saurii, 

 must be regarded as remnants of the primitive intermuscular liga- 

 ments. The obliquus externus generally takes its origin from a 

 large portion of the thorax ; in the Reptilia it is divided into several 

 layers. 



In the Amphibia the transversus abdominis is a large muscle, 

 as it is also in all Reptilia except the Ophidii, where it is absent. It 

 extends as far forward as the thoracic region. In Birds it extends 

 as far as the hinder edge of the sternum, but in Mammals it has a 

 wider area. 



The rectus abdominis appears to be the proportionately least 

 altered portion of the primitive musculature ; its fibres retain their 

 primitive course, and its inscriptiones tendmeas are remnants of its 

 primitive septa. It generally extends from the sternum to the 

 pelvis in the Amphibia, but when the sternum is shortened it is 

 continuous with the sterno-hyoid (Amphibia). 



In the Crocodilini the transverse tendinous bands are ossified, 

 and form the so-called " abdominal ribs." The M. pyramidalis 

 must also be regarded as part of the recti abdominis ; it is found in 

 the Salamandrina, Crocodolini, Ratitse, and also in many Mammals. 

 In the Monotremata and Marsupialia it is largely developed. So 

 much so, indeed, that it nearly reaches to the sternum, and so covers 

 the rectus ; it has its origin in one edge of the marsupial bone. 



§ 375. 



The branchial skeleton of Fishes is provided with a special 

 system of muscles, which is repeated between each of its segments. 

 As the primary pieces of the jaw also belong to this skeleton, their 

 muscles must be regarded as differentiations of the muscular system 



