ELECTEIC ORGANS OF VEETEBEATA. 499 



below the vertebral column ; this is first seen in Eeptiles ; it 

 generally commences within the thoracic cavity, and extends along 

 the neck up to the skull. It breaks up into several portions, which 

 are distiuguished as longus colli, or longus capitis, according to the 

 point of insertion. 



Another subvertebral system of muscles appears to lead to the 

 formation of the Diaphragm. This arrangement does not obtain 

 in Fishes, and it is doubtful whether the separate bands of muscle, 

 which embrace the oesophagus in the Amphibia, can be looked upon 

 as forming a rudimentary diaphragm. Among the Reptilia, the 

 Chelonii have a muscular layer over the lamella of the peritoneum, 

 which encloses the lungs ; this takes its origin partly from the 

 centra of the vertebras, and partly from the rib-like transverse 

 processes. In the Crocodilini there is no diaphragmatic muscle, for 

 it is not possible to regard the highly-developed peritoneal muscu- 

 lature as being a formation of this kind, as it has its origin in the 

 anterior wall of the pelvis. There are indications of a muscular 

 investment of the lungs in Birds ; it is best developed in Apteryx. 



It is in the Mammalia only that there is a well-developed 

 diaphragm forming a partition between the thoracic and abdominal 

 cavities. The oblique direction, taken by the muscle in the Eeptilia 

 and Aves, is necessarily converted into a transverse one. The 

 muscular portions arise in part from the vertebral column, and in 

 part from the ribs ; they pass into a centrum tendineum, which is 

 occasionally absent (Delphinus). 



Humphry, G. M., Observations in Myology. Cambridge and London, 1872. — 

 Furbringer, M., Vergl. Anat. der Schnltermuskeln. Jen. Zeitschr. VII. VIII. 

 Morphol. Jahrb. II.— De Man, Vergl. myolog. u. nenrolog. Stud. Leiden, 

 1873. — Vetter, B., Vergl. Anat. der Kiemen. n. Kiefermuskulatur der 

 Fische. Jen. Zeitschr. VIII. 



Electric Organs. 



§ 378. 



The so-called electric organs are special apparatuses which are 

 found in a very few Fishes; they are of importance, from an 

 anatomical point of view, in consequence of the large masses of 

 nerves which end in them, and from a physiological point of view, in 

 consequence of the development of electricity in them. The endings 

 of the nerves have very much the same relations as have the ends 

 of motor nerves in muscular fibres, while there are many points in 

 the development of these organs which point to their having had 

 their origin in metamorphosed muscles. There is sufficient reason, 

 therefore, for regarding these organs as belonging to the muscular 

 system, although we do not yet know anything of their earlier 

 stage, in which they probably appeared as muscles. 



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