46 NATURAL HISTORY OF VERTEBRATES. 



upper modified part of the wind-pipe, which is the organ of voice in most of the 

 higher vertebrates ; and of the syrinx, the vocal organ of the birds. The latter is 

 situated at the bifurcation of the trachea, and consists of vibrating folds of mucous 

 membrane with a moi-e or less developed resonating apparatus, in adaptation to which 

 certain of the tracheal and bronchial cartilages are altered. Such is also essentially 

 the structure of the larynx, where folds of mucous membrane — the vocal cords — are 

 set in motion by the current of air passing between them, and are adjusted by certain 

 muscles attached to altered cartilages, here known as thyroid, cricoid, and arytenoid. 

 The resonating apparatus varies much in different forms ; perhaps the most remarkable 

 varieties are the huge vocal sacs of the bull-frog, and those of the howling monkeys. 

 There is always a close connection between the larynx and the hyoid-bone, which is 

 established by means of ligaments. 



Thyeoid and Thymus Glands. 



Locally closely related to the respiratory organs, as well as developmentally con- 

 nected with this part of the alimentary tract, are two so-called glands (thyroid and 

 thymus), which, however, functionally belong to the vascular system. 



These structures are common to all vertebrates, and are formed by modification of 

 the epitlielium of the gill-clefts, the former on the ventral, the latter originally on the 

 dorsal aspect. They retain this position in the fishes, but in the air-breathing verte- 

 brates, with the disappearance of the clefts, they are subject to considerable variation 

 in this respect. In man, the thyroid lies at either side of the wind-pipe, the thymus 

 behind the breast-bone. Originally of secretory function (at any rate in the case of 

 tlie thyroid) these structures have gradually become subservient to the lymph-vascular 

 system, and have thus undergone a change of function. The thyroid persists through- 

 out life in functional activity, while the thymus undergoes degeneration soon after 

 birth. 



THE VASCULAR SYSTEjAI. 



Under this heading we have to consider the canals and spaces in which are con- 

 ducted two of the most important fluid constituents of the body — the blood and 

 the lymph. Both of these constituents are composed of a fluid ' plasma,' containing 

 floating corpuscles, and it is the n.iture of these which renders the two fluids differ- 

 ent. In the blood there are two sorts of corpuscles, — the one laden with a coloring 

 matter known as htemoglobin, which can combine with oxygen in the respiratory 

 organs and part with it to the tissues ; the other destitute of coloring matter, and 

 much less constant in form. It is to the former, or red corpuscles, that the blood owes 

 its color, and this varies with the amount of oxygen present in the corpuscles, that is 

 to say, with the degree of aeration of the blood. The colored corpuscles are nearly 

 constant in size and form for each species; with rare exceptions they are small and of 

 circular outline in the Mammalia, but larger, oval, and nucleated in the other classes. 



In accordance with the tendency to change their sliape, and consequently to move, 

 the colorless blood cells or leucocytes are frequently spoken of as amoeboid or wan- 

 dering cells. They are identical with the lymph corpuscles, and play a very different 

 part in the economy from the colored elements. Not only do they retain the power 

 of independent locomotion, but they can engulf food-particles, grow, and reproduce, 

 themselves by division. Unlike the colored elements, they are not confined to the cavi- 

 ties of the vessels, but can effect a passage through the delicate walls of the finest of 



