ENDOSKELETON. AXIMALS. 23 



A. 76 b. Longitudinal section of the left humerus of a Turtle 

 (Chelone my das). There is no medullary cavity, its place 

 being occupied by uniformly dense cancellous tissue. 



A. 76 c. Longitudinal section of the left humerus of a Crocodile 

 (Crocodilus acutus). A medullary cavity is present. The 

 cancellous tissue at the ends of the bone is dense and 

 shows some tendency to a vertical disposition. 



BlKDS = AVES. 



A. 77. The muscles and bones of the hind limb of a Silk-fowl 

 ( Gallus femif/ineus, var. mono), showing the peculiar dark 

 brown colour of the periosteum. 0. (J. 2057. llunterian. 



A. 78. The right tarso-metatarsus of the domestic Fowl, 

 longitudinally bisected, to show the results of the following 

 experiment : 



When the animal was young, the bone was perforated near each 

 extremity, and a small leaden shot was introduced into each hole. 

 After a certain period the animal was killed, and the length of the 

 bone was found to have been increased to three inches and ten 

 lines ; but the distance between the shots, which has now reached 

 the medullary cavity, was exactly the same as when first intro- 

 duced. [The original note of the experiment is preserved in the 

 bottle.] 



" And here I must observe, that a bone does not grow in 



all its parts, that is, it does not grow by addition of new particles 

 among those already arranged, or in their interstices, but by tbe 

 addition of parts lengthways or sideways of the bone. This 1 

 proved by exposing the bones of young animals, and boring holes 

 in them, which were prevented from being obliterated by fixing 

 pieces of leaden shot in them ; these bones were examined a con- 

 siderable time after, when, although the bones bad considerably 

 grown, tbe holes were exactly at the same distance from eacb 

 other." John Hunter, MS. Lectures. 



0. C. 189. Hunterian. 



A. 79. The left tarso-metatarsus of the domestic Fowl, upon 

 which the following experiment was performed : 



Two small holes were made by cauterization near the ex- 

 tremities of the bone ; tbe length of the bone at that time being 



