THE RESPIRATORY GROANS. 91 



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 Grocodilia, among Reptilia, that an accumulation 

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

 is apparent, as a Lymphatic gland, in the mesentery. Birds 

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

 malia, they are found, not only in the mesentery, but in many 

 parts of the body. 



The Spleen is substantially a lymphatic gland. The Thy- 

 mus — a glandular mass with an internal cavity, 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 neighborhood 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 Suprarenal 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 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 Lymph 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 hranchicB 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 branchim 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 different trunks which carry the aSrated 

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

 vertebrated animals with exclusively branchial respiration 

 have the heart filled with venous blood. 



In the early life of many branchiated Vert^rata, the bran- 



