172 



THE SHEPHERD'S MAKUAL. 



At twenty-seven months, the temporary molars are changed, and per- 

 manent ones appear. 



At thirty months, there are six permanent incisors, (a, a, fig. 64). 



At thirty-six to forty-two months, there are eight permanent inci- 

 sors, (a, a, fig. 65). 



It has been decided in an English court of law, that a lamb be- 

 comes a sheep when the first pair of permanent incisors have ap- 



Ct 



Fig. 62. Fig. 63. 



peared. When the mouth is full toothed, the sheep is considered 

 as mature, or full grown, when the teeth begin to show signs of 

 wear, the sheep is " aged." 



The composition of the bones of the sheep and of the marrow 

 con tamed in their cavities differs in no respect from that of the 

 bones and marrow of other domestic animals. Bone consists of 

 animal matter and earthy salts ; usually in the following propor- 

 tion, viz : phosphate of lime, 55 per cent ; carbonate of lime, 4 per 

 cent ; phosphate of magnesia, 3 per cent ; soda, potash, and com- 



Fig, 64. 



Fig. 65. 



mon salt, (chloride of sodium), 3 per cent, and 35 per cent of gela- 

 tine. These proportions differ to some extent with the kind of 

 bone, its solidity, and the age and condition of the animal from 

 which it is taken. The bones of the skeleton are joined together 

 by means of cartilages, or ligaments, which form what are known 

 as articulations. Where the bones move one upon another, their 

 ends, or the parts brought in contact, are covered with cartilage. 

 Every bone is enveloped in a highly sensitive lining or membrane 

 called the periosteum. Although the bones seem to be solid and 

 insensible matter, chiefly composed of earthy salts, yet they are 

 highly sensitive, and are formed of tissue which is penetrated by 



