COMPARATIVE ANATOMY. 



,have stemmata, besides their compound 

 eyes. 



The internal structure has hitherto 

 been investigated only in the large poly- 

 edrous eyes. The back of the cornea 

 (which is the part divided in front into 

 the hexagonal surfaces, called in French, 

 facettes) is covered with a dark pigment : 

 behind this are numerous white bodies, 

 of an hexagonal prismatic shape, and 

 equal in number to that of the facettes 

 of the cornea. A second coloured mem- 

 brane covers these, and appears to receive 

 the expansion of the optic nerve. 



Further investigation is, however, re- 

 quired, in order to shew how these eyes 

 enable the insect to see ; and to deter- 

 mine the distinction between two such 

 very different organs. 



MUSCLES. 



The nature and objects of the present 

 work render it impossible for us to enter 

 into the details of comparative myology ; 

 we shall therefore restrain our remarks to 

 one or two subjects. 



The differences which we discern in 

 the muscles of the lower extremity, be- 

 tween man and the other mammalia, 

 arise out of that characteristic feature, 

 which so strikingly distinguishes man 

 from all other animals, viz. his erect sta- 

 ture. The most minute investigation of 

 this subject will shew us that the erect 

 position belongs to man only ; and there- 

 by confirms the elegant observation of the 

 Roman poet : 



Pronaque cum spectent animalia cetera 



terram, 



Os homini sublime dedit ; ccelwnque luerl 

 Jussit ; et erectos ad sidera toilers vultus. 



In order to enable any animal to pre- 

 serve the erect position, the following 

 conditions are required. 1st. That the 

 parts of the body should be so disposed 

 as to admit of being maintained with 

 ease in a state of equilibrium. 2dly. That 

 the muscles should have sufficient power 

 to correct the deviations from this state. 

 3dly. That the centre of gravity of the 

 whole body should fall within the space 

 occupied by the feet; and lastly, That 

 the feet themselves should have a broad 

 surface, resting firmly on the ground, and 

 should admit of being in a manner fixed 

 to the earth. All these circumstances 

 ar? united in the necessary degree in man 

 only. 



The broader the surface included by 



the feet, the more securely will the line of 

 gravity rest within that surface. The feet 

 of man are much broader than those of 

 any animal, and admit of being separated 

 more widely from each other. The sources 

 of the latter prerogative reside in the su- 

 perior breadth of the human pelvis, and in 

 the length and obliquity of the neck of 

 the femur, which, by throwing the body 

 of the bone outwards, disengage it from 

 the hip-joint. 



The whole tarsus, metatarsus, and toes, 

 rest on the ground in the human subject, 

 but not in other animals. The simiae, and 

 the bear, have the end of the os calcis 

 raised from the surface ; while, on the 

 contrary, it projects in man, and its pro- 

 minent portion has a more important share 

 in supporting the back of the foot. The 

 exterior margin of the foot rests chiefly 

 on the ground in the simiae ; which cir- 

 cumstance leaves them a freer use of their 

 thumb and long toes in seizing the branch- 

 es of trees, &c. ; and renders the organ so 

 much the less adapted to support the bo- 

 dy on level ground. 



The plantaris muscle, instead of termi- 

 nating in the os calcis, expands into the 

 plantar fascia in the simiae ; and in other 

 quadrupeds it holds the place of the flexor 

 brevis, or perforatus digitorum pedis, pass- 

 ing over the os calcis in such a direction, 

 that its tendons would be compressed, and 

 its action impeded, if the heel rested on 

 the ground. 



The extensors of the ankle joint, and 

 chiefly those which form the calf of the 

 leg, are very small in the mammalia, even 

 in the genus simiae. The peculiar mode of 

 progression of the human subject suf- 

 ficiently accounts for their vastly superior 

 magnitude in man. By elevating the os 

 calcis, they raise the whole body in the 

 act of progression ; and, by extending the 

 leg on the foot, they counteract that ten- 

 dency which the weight of the body has 

 to bend the leg in standing. 



The thigh is placed in the same line with 

 the trunk in man ; it always forms an angle 

 with the spine in animals, and this is often 

 even an acute one. The extensors of the 

 knee are much stronger in the human sub- 

 ject than in other mammalia, as their dou- 

 ble effect of extending the leg on the 

 thigh, and of bringing the thigh forwards 

 on the leg, forms a very essential part in 

 the human mode of progression. 



The flexors of the knee are, on the 

 contrary, stronger in animals, and are in- 

 serted so much lower down in the tibia 

 (even in the simiae) than in the human 

 subject, that the support of the body on 



