150 HISTOLOGY 



times almost comes to rest. These form the so-called blood glands, and 

 in them the blood is renovated by the removal of some of its old parts or 

 the addition of other new ones or both. There are several kinds of these 

 organs and they will be treated of later. 



Owing to the homogeneous histological structure of the circulatory 

 organs in the various groups of animals, we shall study the detail of these 

 organs by going through the individual system of several typical forms. 

 It must be held in mind that the walls of these organs show a strong 

 analogy based on the physiological (which are here mechanical) needs of 

 the vessels. The blood fluid must be confined to the channels, and this is 

 usually done by the single inner layer of cells, the intima. In some forms 

 the intima is formed, not by the cells themselves, but by a cuticle which is 

 the product of these cells (see the paragraphs on the lobster and Imperia- 

 lis larva). The intima may alone confine the blood stream, or if the 

 pressure is too great, it may be reenforced by the connective-tissue cells 

 that immediately surround it. These cells develop their connective tissue 

 as fibrils or plates or webs with which they bind and hold the vessel in- 

 tact when the blood presses on its walls. Again, these primitive meso- 

 derm cells may develop into muscle cells that surround the channel and 

 by their contractile strength cause it to pulsate and drive the blood on its 

 course. The arrangement of these three classes of tissues to form the 

 wall of the vessel falls, naturally, into layers, the so-called coats of 

 the blood vessels. Each kind of coat usually has a particular posi- 

 tion with reference to the lumen. This position, however, is some- 

 times changed in the several groups for no apparent reason. 



All these cells and the tissues that they form were probably not cells 

 that were bound in the course of their development to become so special- 

 ized, but, as far as can be told, they were such of the connective-tissue 

 cells as happened to be in the course of the developing blood channel as 

 it pushed its way among them, and were developed in response to the 

 needs of the vessels. Any of these connective-tissue elements would 

 probably do the same if the blood vessel came their way, especially in 

 the embryonic stages of the organism. This view is open to debate, 

 however, until observation has brought proof. 



We shall study the walls of the blood vessels in a few typical forms 

 to see what variation is found among them from a histological point of 

 view. Some Turbellarian as a primitive form ; the worm Cerebratulus, 

 the earthworm Allolobophora, the mollusk Unio, the mollusk Octopus, 

 the crustacean Homarus, and an insect, Imperialist Amphioxus with 

 reference to a Tunicate, and the Vertebrate, man, with reference to a 

 salamander, will cover the ground satisfactorily. Forms lower than the 

 Nemerteans seldom possess a circulatory system. 



The internal tissue of a Turbellarian worm is a loose aggregate of 



