562 W. N, F. WOODLAND. 



layers : an inner elastic and muscular layer covered internally by the 

 squamous epithelium lining the bladder cavity, a middle conjunctive 

 and vascular layer, and an outer fibrous layer. At the periphery or 

 edge of the oval there is developed a special band of circular smooth 

 muscle fibres, which by contraction can lessen and obliterate altogether 

 the area of the oval exposed to the gases in the general bladder cavity. 

 Over this area, limited externally by the circular band just mentioned, 

 the inner layer is quite absent, only the squamous epithelium being 

 present, and this latter in consequence abuts directly on the middle 

 layer, in which, in the oval area, the capillary system is much 

 developed.* In the region of the oval, therefore, the gases contained 

 in the bladder can come into very close contact (only separated by the 

 squamous epithelium) with the numerous capillaries of the oval 

 contained in the middle layer. Attached to the edge of the oval, 

 immediately external to the circular muscle band, are numerous radial 

 muscle fibres (belonging to the inner layer surrounding the oval), 

 the function of which is to act in opposition to the circular band 

 and enlarge the oval area. The foregoing statements and the mode 

 of action of the oval, according to Eeis and Nusbaum, are illustrated 

 by Figures 1-4 (devised from the statements and diagrams of these 

 authors). It will be seen that, according to these authors, the closure 

 of the oval, in the fishes studied by them, is effected by the simple 

 contraction of the circular muscle band (the radial muscles slackening), 

 the squamous epithelium being thereby raised from contact with the 

 blood-vessels and separated from them by the muscles. I presume 

 that these statements of Nusbaum and Eeis are based upon the study 

 of actual sections of closed and open ovals ; otherwise I should doubt 

 their accuracy, because this mode of action of the oval is quite unlike 

 that of the oval in the Pollack and the Mullet, because I find it diffi- 

 cult to believe that the squamous epithelium ever becomes separated 

 from the capillary plexus in the manner asserted, and finally because, 

 if Tracy (Anat. Anzeiger, 1911) is correct in his interesting view 

 that the oval is homologous with the posterior chamber of the Carp 

 bladder and the distal part of the ductus pneumaticus of Physostomi 

 (Fig. 7), these statements are improbable a priori. It is evident that 

 if the edge of the oval is homologous with the circular edge of the 

 septum separating the anterior and posterior chambers of the Carp or 

 Siphonostoma bladder, then it might naturally be anticipated that the 



* The so-called " wundernetz " — a bad term, since this special capillary development 

 has nothing to do with the rete mirabile duplex situated on the artery and vein supplying 

 the gas-gland (vide my Anat. Anzeiger paper already mentioned and Proc. Zoo/. Soc, 

 Lond., 1911, p. 183). 



