SECRETION 361 



their own defibrinated blood, but are unaffected by extract of leeches. A dry 

 commercial preparation of great activity, known as " hirudin," which is made by 

 the method described by Franz (1903), is much used. Franz regards the active 

 constituent as being a kind of albumose. Abel finds that it is in colloidal solution 

 in water. 



Silk and the similar substance of spiders' webs are very interesting products of 

 secretion. They are of protein nature and are formed in the liquid state by special 

 glands. The liquid is forced through fine apertures and rapidly sets in contact with 

 air. By opening up the silkworms, a considerable quantity of the liquid secretion 

 can be obtained, which can then be used for making fibres of greater thickness than 

 those made by the insects themselves. These threads are of great strength, and 

 are valuable for fish lines, ligatures, etc. 



The Gas Bladder of Fishes. Since the substance of which the body of fishes 

 is composed is of a higher specific gravity than that of sea water, it is obviously of 

 advantage to them to possess a float, containing gas which, present in the 

 appropriate amount, will reduce their weight to that of an equal bulk of water, 

 thus removing the necessity of muscular movement in order to keep themselves from 

 sinking. An organ of this kind actually exists in the teleosts. But it is clear 

 that the gas will be compressed as the fish sinks, thus becoming of a greater 

 specific gravity, and more must be produced to restore proper compensation. 

 Conversely, when the fish rises again, the gas will expand and displace other 

 tissues, as in fact happens when deep sea fish are brought to the surface rapidly. 

 In some fish, there is a duct to the exterior, via the oesophagus, which can be 

 opened to allow of escape of gas ; and in others, where the duct has become solid, in 

 a special region, the "oval," the wall of the gas bladder has the power of absorbing 

 the gas. This oval can be shut off from communication with the gas bladder when 

 nob required. At first sight, it might seem strange that the gas is found to consist 

 almost entirely of oxygen, but if it has to be secreted and absorbed, the advantages 

 are obvious. Oxygen can easily be obtained from oxyhsemoglobin and can be used up 

 either by combination with reduced haemoglobin or by oxidation of some reducing 

 substance. Woodland (1911, 1 and 2) has made an interesting investigation of 

 the structure and physiology of the gas bladder, and the reader is referred to his 

 papers, which contain also a list of other papers. A remarkable vascular organ is 

 found in the course of the blood vessels supplying the gland in the gas bladder which 

 secretes oxygen. This organ is what is called a rete mirabife, in which the artery 

 divides into a number of fine arterioles, which lie closely, side by side, with the 

 corresponding finely divided veins carrying blood from the gas gland. These 

 vessels do not join each other, but allow of free interchange of diffusible 

 constituents of the blood, and have, obviously, an important function in relation to 

 the secretion of gas. It would seem that some chemical substance must be 

 produced in the gland cells, which is not desirable in the general circulation. As 

 this substance returns in the veins, it diffuses out into the blood of the arterioles 

 in the rete and is, for the most part, sent back to the gland. It is probably 

 something which enables oxyhsemoglobin to give up its oxygen readily ; that there 

 are substances of this kind will be seen in Chapter XXI. It is not a hsemolysin, 

 since there is no evidence of the presence of such a substance, nor is there haemoglobin 

 in the cells of the gland itself. The gas must therefore be given off by the 

 corpuscles in much the same way as to other tissue cells. The preparations of 

 Woodland show that the gas gland possesses large cells, similar in appearance 

 to those of a typical secreting gland, and, according to Bohr (1894), the vagus 

 nerve supplies secretory fibres, since, after section of the intestinal branch of this 

 nerve, no further secretion of gas takes place, even when the bladder is emptied of 

 gas, a procedure always resulting normally in renewed formation of oxygen. 



The protozoan, Arcella, forms bubbles of gas and raises itself to the surface of 

 water by this means. According to Bles (1910), these bubbles consist of oxygen. 

 The stimulus to secrete the oxygen bubble appears to be, curiously enough, the 

 want of oxygen in the depths of the pond water; the animal thus floats itself to 

 the surface. In order to sink again, the animal must absorb the oxygen, since it 

 cannot escape to the air, owing to the shell on the animal. 



