COMPARATIVE ANATOMY. 



One of the flexor tendons of the toes of 

 birds, (produced from a muscle which 

 comes from the pubis) runs in front of the 

 knee ; and all these tendons go behind the 

 heel : hence the flexion of the knee and 

 heel produces mechanically a bent state 

 of the toes, which may be seen in the dead 

 bird ; and it is by means of this structure 

 that the bird is supported, when roosting 1 , 

 without any muscular action. 



This circumstance of the flexion of the 

 toes accompanying 1 that of the other joints 

 of the lower extremity of birds was long- 

 ago observed by Borelli, and attributed 

 by him to the connection which the flex- 

 ors of the toes have with the upper parts 

 of the limb, by which they are mechani- 

 cally stretched when the knee is bent. 

 This explanation has been controverted 

 by Vicq d'Azyr, and others, who have 

 referred the effect to the irritability of 

 the muscles. The opinion of Borelli ap- 

 pears, notwithstanding, to be well found- 

 ed ; for not only the tendon of the ac- 

 cessory flexor passing 1 round the knee, 

 but the course of the flexor tendons over 

 the heel, and along- the metatarsus, must 

 necessarily cause the contraction of the 

 toes when either of these joints is bent ; 

 and if the phenomenon was not produced 

 on mechanic principles, it would be im- 

 possible for birds to exhibit it during 

 sleep, which they do, or to prove the ef- 

 fect on the limb of a dead bird, than 

 which nothing- is more easy. The utility 

 of this contrivance is great in all birds, 

 but particularly so in the rapacious tribe, 

 which, by this means, grasp their prey 

 in the very act of pouncing- on it ; and 

 it is still more necessary to those birds 

 which perch or roost during their sleep, 

 as they could not otherwise preserve their 

 position, when all their voluntary powers 

 are suspended. 



URINARY ORGANS. 



The structure of the kidney in the 

 mammalia displays two very opposite 

 varieties, which may be called the simple 

 and the conglomerated kidneys. In the 

 former there is a single papilla, which 

 is surrounded by an exterior crust of 

 cortical substance. This is the case in 

 all the ferae, and in many rodentia. The 

 other kind of kidney consists of an ag- 

 gregation of small kidneys, connected by 

 cellular substance. It appears that this 

 form of the gland is found in all those 

 mammalia which either live in or fre- 

 quent the water. I have observed it in 

 the seal and porpoise, where the small 



VOL. III. 



kidneys are extremely "numerous, and 

 send branches to the ureter without 

 forming- a pelvis. Mr. Hunter states 

 that it belongs to all the whales. (" Phi- 

 los. Transact. 1807, pt. 2.") The otter 

 has the same structure ; but its small kid- 

 neys are not so numerous as in the ani- 

 mals above-mentioned. (" Home, of the 

 sea-otter (lutra marina,) Philos. Trans. 

 1796, pt. 2.") It is remarkable that the 

 brown bear (ursus arctos,) which lives on 

 land, should have this structure as well 

 as the white polar bear (ursus >mariti- 

 mus,) which, inhabiting- the coasts and 

 floating- ice of the northern regions, 

 spends much of its time in the water, 

 Mr. Hunter concludes, that it is because 

 nature wishes to preserve an uniformity 

 in the structure of similar animals. But 

 the badger, (ursus meles,) which is a 

 very similar animal, has the uni-lobu- 

 lar kidney. The number of small 

 kidneys in the bear is 50 or 60, and 

 it appears that each consists of two pa- 

 pillae. 



The kidneys of birds form a double 

 row of distinct, but connected glandular 

 bodies, placed on both sides of the lum- 

 bar vertebrae, in cavities of the ossa in- 

 nominata. The urinary bladder does not 

 exist in this whole class, and the ureters 

 open into cloaca. 



Animals of the genus testudo and rana 

 have a large bladder in the situation of 

 the urinary receptacle of other animals. 

 This is double in many of the frogs, pro- 

 perly so called. These bags are repre- 

 sented both by Blumenbach and Cuvier 

 as urinary bladders ; but Townson has 

 already shown, that in the frog and toad 

 they have no connection with the ure- 

 ters, which open at the back of the 

 rectum, while those receptacles termi- 

 nate on the front of the intestine. 

 ("Tracts and Observations," p. 66. fig. 

 3.) The writer of this article has ob- 

 served the same structure in a male and 

 female tortoise. 



ORGANS OF GEKERATIOJf. 



The nature of generation, which is the 

 greatest mystery in the economy of liv- 

 ing bodies, is still involved in impenetra 

 ble obscurity. The creation of a living 

 body, that is, its formation by the union 

 of particles suddenly brought together, 

 has not hitherto been proved by any di- 

 rect observation. The comparison of this 

 process to that of crystallization is found- 

 ed in a false analogy ; crystals are formed 

 of similar particles, attracting each other 



T t 



