58 MANUAL OF HISTOLOGY. 



juice, saliva, semen, and milk, as well as in the juices of organs. Again, 

 it invariably accompanies histogenic substances, as has been already men- 

 tioned, and appears with the same in the tissues and fluids of our body. 

 It is present in bone in large quantities as the chief constituent of the 

 hard material of this tissue known as bone earth. But in the enamel of 

 the teeth, the hardest substance in the whole body, it exists in still 

 greater quantity. 



Phosphate of calcium must be regarded as an indispensable element of the 

 tissues of the body ; we must, therefore, ascribe to it histogenic properties. 



CARBONATE OF CALCIUM, CaC0 3 . 



This, like the preceding salt, occurs in the amorphous condition as 

 hardening material in bones and teeth, but only in small amount. Be- 

 sides this, it is met with in some of the animal fluids, as, for instance, 

 the saliva, and in alkaline urine. It is also found in a crystalline form 

 in the internal ear of man, constituting what are known as otoliths. It 

 is met with more frequently still, however, in this state, in the bodies of 

 the lower vertebrates, as, for instance, in frogs, deposited upon the mem- 

 branes of the brain and spinal cord, and also on the anterior aspect of 

 the spinal column, about the place of exit of the spinal nerves. 



Otoliths (fig. 37) are small crystals of short, thick, columnar form, com- 

 binations of rhombohedrons and hexagonal prisms in their fundamental 

 figure ; among them may also be found pure rhombohedrons, or scaleno- 

 hedrons. 



The question as to what it is that retains carbonate of calcium in solution 

 in the fluids of the body, has not yet been answered satisfactorily. It 

 seems probable, however, that the carbonic acid diffused through the latter 

 is the real solvent for the salt. Any other physiological purpose besides 



that of a hardening medium of the 

 second class, has not as yet been 

 recognised for carbonate of calcium as 

 it appears in the bodies of the higher 

 animals. 



Carbonate of calcium is partly taken 

 up as such from without, and is partly 

 formed in the body by the develop- 

 ment of carbonic acid as a decom- 

 position product (see above). 

 ^f^ar * ^HX 



CHLORIDE OF CALCIUM, CaCl 2 . 



Is of but subordinate significance, 



Fig. 37. Otolithsconsistingofcarbonateof calcium, n^^ has as Vet been met with in 



the gastric juice only (Braconnot). 



FLUORIDE OF CALCIUM, CaFL 2 . 



This salt is found in the enamel of the teeth and in small quantities in 

 bone also ; traces of it are also met with in the blood, milk, and urine, 

 saliva, and bile, and in the hairs (Nickles). Fluoride of calcium is 

 taken up from without as such. 



42. 



Magnesium Compounds. 

 Magnesium appears under similar circumstances, combined with phos- 



^ 



