329 



pear an exception to the general rule respecting buds and roots ; but 

 the author observes, that the tuber differs but little from a branch 

 which has dilated instead of extending itself. The runners, which 

 give existence to the tubers beneath the soil, are very similar in or- 

 ganization to the stem of the plant; and if exposed, readily emit 

 leaves, and perform all the functions of the stem ; and, on the other 

 hand, Mr. Knight has shown, in a former memoir, that the buds on 

 any part of the stem may be made to produce tubers similar to those 

 formed beneath the soil ; but he has never, under any circumstances, 

 been able to obtain tubers from the fibrous roots of the plant. 



Many naturalists have imagined the fibrous roots of all plants to 

 be of annual duration only, because those of bulbous and tuberous 

 plants certainly are so ; but Mr. Knight observes, that the organi- 

 zation of trees is extremely different ; and he has not found any por- 

 tion of their roots to be deciduous. 



On the Nature of the intervertebral Substance in Fish and Quadrupeds . 

 By Everard Home, Esq. F.R.S. Read February 23, 1809. [Phil. 

 Trans. 1809,^. 177.] 



The author, having observed a new species of joint in the Squalus 

 maximus of Linnaeus, takes occasion to trace the successive gradations 

 of a similar structure, through various kinds of fish, to the more re- 

 mote resemblance to be found in quadrupeds and in men. 



In the Squalus each joint of the spine approaches, in some mea- 

 sure, to that which is termed the ball and socket joint, as a concave 

 surface of each vertebra is applied to a ball ; but the ball, in this in- 

 stance, is not, as in other cases, a smooth surface covering a solid 

 bone, but a collection of fluid contained in a bag that is nearly sphe- 

 rical, round which the concave surfaces of the vertebrae are moved. 



In a fish of thirty feet in length, the diameter of the body of one 

 of the largest vertebrae measured seven inches ; the quantity of fluid 

 in one of the cavities amounted to three pints ; the ligamentous sub- 

 stance, which unites the vertebrae, being nearly one inch in thickness, 

 externally very compact and elastic, but internally possessed of but 

 little elasticity. 



The elasticity of these ligaments preserves the straitness of the 

 spine when it is not acted upon by the muscles, or by other external 

 force ; and though the extent of motion, in any one joint, must be 

 small, their number affords considerable latitude of motion. 



Since the vertebrae, in other fish, are found with concavities in each 

 surface, it was natural to expect a corresponding resemblance in the 

 intervertebral structure ; and in the skate this was found to be the 

 case, and the cavity nearly spherical, as in the Squalus. In the com- 

 mon eel it is more oblong, the longitudinal diameter exceeding the 

 transverse one by about one third. 



In the sturgeon the structure varies considerably, as the cavities 

 communicate with each other by apertures through the bodies of the 

 vertebrae, which in this fish are cartilaginous rings, connected toge- 



