THE MUSCULAR SYSTEM. 679 



Its further history is very interesting. In my original account 

 of this cavity (No. 292, p. 208) I stated my belief that its walls 

 gave rise to the eye-muscles, and the history of this process has 

 been to some extent worked out by Marshall in his important 

 memoir (No. 509). 



Marshall finds that the ventral portion of this cavity, where 

 its two halves meet, becomes separated from the remainder. 

 The eventual fate of this part has not however been followed. 

 Each dorsal section acquires a cup-like form, investing the 

 posterior and inner surface of the eye. The cells of its outer 

 wall subsequently give rise to three sets of muscles. The middle 

 of these, partly also derived from the inner walls of the cup, 

 becomes the rectus internus of the eye, the dorsal set forms the 

 rectus superior, and the ventral the rectus inferior. The obliquus 

 inferior appears also to be in part developed from the walls of 

 this cavity. 



Marshall brings evidence to shew that the rectus externus (as 

 might be anticipated from its nerve supply) has no connection 

 with the walls of the premandibular head-cavity, and finds that 

 it arises close to the position originally occupied by the second 

 and third cavities. Marshall has not satisfactorily made out the 

 mode of development of the obliquus superior. 



The walls of the cavities, whose history has just been re- 

 corded, have definite relations with the cranial nerves, an account 

 of which has already been given at p. 461. 



Head-cavities, in the main similar to those of Elasmo- 

 branchii, have been found in the embryo of Petromyzon (fig. 45, 

 hc\ the Newt (Osborn and Scott), and various Reptilia (Parker). 



BIBLIOGRAPHY. 



(507) G.M.Humphry. " Muscles in Vertebrate Animals." Journ. of Anat. 

 and Phys., Vol. vi. 1872. 



(508) J. Miiller. " Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie 

 u. Myologie." Akad. Wiss., Berlin, 1834. 



(509) A. M. Marshall. "On the head cavities and associated nerves of 

 Elasmobranchs." Quart. J. of Micr. Science, Vol. XXI. 1881. 



(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere." 

 Sitz. d. Oberhessischen Gesdlschaft, 1873. 



(511) A. Schneider. Bdtriige z. -vergleich. Anat. it. Entwic k. d. Wirbelthiere. 

 Berlin, 1879. 



Fwfcalso Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc. 



