THE RESPIRATORI ORGANS. 91 



Accumulations of indifferent tissue in tlie vralls of some of 

 the lymphatic sinuses are to be met Avith in Fishes ; but it is 

 only in the Grocodilia^ among Heptilla^ tliat an accumulation 

 of such tissue, traversed by lymphatic canals and blood-vessels, 

 is apparent, as a LympJiatio gland^ in the mesentery. Birds 

 possess a few glands in the cervical region ; and, in Mam- 

 raalia^ tliey are found, not only in the mesentery, but in many 

 parts of the body. 



The Spleen is substantially a lymphatic gland. The Tliy- 

 mus — a glandular mass with an internal cavity, but devoid of 

 any duct — which is found in all Vertehrata except Amj^hioxus, 

 appears to belong to the same category. It is developed in 

 the neighborhood of the primitive aortic arches, and is double 

 in most of the loAver Vertebrata, but single in JIammalia. 



The nature of two other " ductless glands," the Thyroid 

 gland and the Suprarenal capsules^ which occur very widely 

 among the Vertebrata, is by no means Avell understood. 



The thyroid gland is a single or multiple organ, formed of 

 closed follicles, and is situated near the root of the aorta, or 

 the great lingual, or cervical, vessels which issue from it. 



The suprarenal capsules are follicular organs, often abun- 

 dantly supplied with nerves, which appear to occur in Fishes, 

 and are very constant in the higher Vertebrata, at the anterior 

 ends of the true kidneys. 



The Ijympli Corpuscles^ which float in the plasma of the 

 lymphatic fluid, always resemble the colorless corpuscles of 

 the blood. 



The Respiratory Organs. — ^Vertebrated animals may pos 

 sess either branchicB for breathing the air contained in water, 

 or lungs for atmospheric respiration; or they may possess 

 both kinds of respiratory organs in combination. 



Except in Amphioxus^ the branchice are always lamellar, 

 or filamentous, appendages of more or fewer of the visceral 

 arches ; being sometimes developed only on the proper bran- 

 chial arches, sometimes extending to the hyoidean arch, or 

 (as would appear to be the case with the spiracular bran- 

 chiae of some fishes) even to the mandibular arch. The bran- 

 chiae are always supplied with blood by the divisions of the 

 cardiac aorta ; and the difibrent trunks which carry the aerated 

 blood away, unite to form the subvertebral aorta, so that all 

 vertebrated animals Avith exclusively branchial respiration 

 have the heart filled Avith venous blood. 



In the early life of many branchiated Vertebrata^ the bran- 



