244 COMPARATIVE ANATOMY 



placed obliquely in the anterior part of the sinus, not far 

 behind the subciavian artery, from which it receives a branch. 

 Similar but much smaller bodies of discoid shape are placed 

 on the courses of the intercostal arteries throughout the length 

 of the sinus. The function and significance of these bodies is 

 problematical. Recent researches indicate that they are not 

 homologous with the supra-renal capsules of higher vertebrates, 

 as was once supposed, and we may call them the supra-renal 

 bodies. Whatever may be their function and significance they 

 are closely related to the sympathetic system. The first 

 sympathetic ganglion is imbedded in the substance of the first 

 supra-renal body, and the more posterior ganglia are either 

 imbedded in or placed in proximity to the remaining supra- 

 renal bodies. The first sympathetic ganglion is much the 

 largest of the series, and is connected by a network of fine 

 nerve fibres with a large plexus situated in the anterior corner 

 of the posterior cardinal sinus, which in its turn is connected 

 with fibres of the visceral branch of the vagus nerve. Posteriorly 

 the first sympathetic ganglion gives off a number of nerve 

 bundles which constitute the splanchnic plexus, closely attached 

 to the walls of the oesophagus. The remainder of the sym- 

 pathetic ganglia are minute, measuring from f^ths to T T (yt ns of 

 a millimetre in diameter. They are united in an irregular 

 manner with one another by very slender nerves, and are 

 connected with the spinal nerves by fine fibres representing 

 the rami cpmmunicantes of higher forms. 



The organs of special sense may be briefly described. 



The olfactory organs are a pair of spacious sacs, enclosed in 

 the cartilaginous olfactory capsules. The nostril on either side 

 leads into the cavity of the sac, which is subdivided into a 

 number of chambers by vertical partitions or septa, attached 

 on either side of a median partition which runs across the 

 cavity from its posterior and outer, to its anterior and inner 

 wall. The walls of the septa are thrown into smaller folds 

 and are covered by a thick epithelium, in which the olfactory 

 cells are situated. The epithelium is richly supplied by the 

 branches of the olfactory nerves. The eye has the same 

 general structure as that of the frog (vol. i. p. 69), and need 

 not be described at length. Its more remarkable features 

 are the cartilaginous condition of the sclerotic, the flatness 



