THE EESPIEATOET ORGANS. 101 



these sinuses are m\iscular, and contract rhytlimicaUy, so 

 that they receive the name of Lymphatic hearts. The pos- 

 terior pair of these hearts, or non-pidsating sinuses cor- 

 responding -with them, are met with in Beptilia and Aves. 



Accumulations of indifferent tissue in the walls of some 

 of the lymphatic sinuses are to be met with in Fishes ; 

 but it is only in the Crocodilia, among Reptilia, that an 

 accumulation of such tissue, traversed by lymphatic canals 

 and bloodvessels, is apparent, as a Lymphatic gland, in the 

 mesentery. Birds possess a few glands in the cervical 

 region ; and. in Mammalia, they are found, not only in the 

 mesentery, but in many parts of the body. 



The Spleen is substantially a lymphatic gland. The 

 Thymus — a glandular mass with an internal cavitj, but 

 devoid of any duct — which is found in all Vertebrata except 

 Amphioxus, appears to belong to the same category. It is 

 developed in the neighbourhood of the primitive aortic 

 arches, and is double in most of the lower Vertebrata, but 

 single in Mammalia. 



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

 gland and the Sitprarenal capsules, which occur very widely 

 among the Vertebrata, is by no means well understood. 



The thyroid gland is a single or multiple organ, formed 

 of closed follicles, and is situated near the root of the aoi-ta, 

 or the great lingual, or cervical, vessels which issue fi-oni it. 



The suprarenal capsules are foUicular organs, often 

 abtmda-ntly 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 Lymph Corpuscles, which float in the plasma of the 

 lymphatic fluid, always resemble the coloitrless coi-puscles 

 of the blood. 



The Respiratory Organs. — Vei-tebrated animals may possess 

 either branchice for breathing the air contained in water, or 

 lungs for atmospheric respiration ; or they may possess both 

 kinds of respiratory organs in combination. 



