COMPARATIVE ANATOMY. 



must therefore affect the diameterof those 

 tubes by their action. The whole of 

 these muscular fasciculi are surrounded 

 and connected together by a while, uni- 

 form, adipous substance. The transverse 

 ones are not more than a line in thick- 

 ness. If the number of these which ap- 

 pears on a transverse section be ascer- 

 tained, and if those portions of the longi- 

 tudinal fasciculi, which pass from one ten- 

 don to another, be reckoned as separate 

 muscles (for they must have a separate 

 power of action) the whole trunk will 

 contain about thirty or forty thousand 

 muscles, which will account satisfactorily 

 for the wonderful variety of motions 

 which this admirable organ can execute, 

 and for the great power which it is capa- 

 ble of exerting. 



The blowing hole of the whale serves 

 as well for respiration as for the rejection 

 of the water which enters with, the food. 

 In consequence of its situation at the top 

 of the head, it is easily elevated beyond 

 the surface of the sea, while the mouth 

 is usually under water. 



The opening in the bones of the head 

 is divided into two by a partition of bone ; 

 and is furnished with a valve opening 

 outwards. On the side of this open- 

 ing are two membranous bags lined 

 with a continuation of the integuments, 

 and opening externally. The water, 

 which the animal wishes to discharge, is 

 thrown into the fauces, as if it were to 

 be swallowed ; but its descent into the 

 stomach is prevented by the contraction 

 of the circular fibres of the oesophagus. 

 It therefore elevates the valve placed at 

 the entrance of the blowing-holes, and 

 distends the membranous bag, from which 

 it is forcibly expelled by surrounding 

 muscular fibres. 



This apparatus occupies the situation, 

 which in other mammalia is filled by the 

 nose ; which organ, together with the si- 

 nuses of the head, the olfactory nerve, 

 &c. is entirely wanting in these animals. 



ORGAN OF HEARING. 



Some Mammalia have not an external 

 ear, particularly such as live in the wa- 

 ter or under ground. 



Most quadrupeds have a peculiar he- 

 mispherical bony cavity, communicating 

 with the tympanum, and seeming to hold 

 the place of mastoid cells. 



The ornithorhynchus, whose structure 

 is in every respect so anomalous, has only 

 two ossicula auditus. 



The cochlea, which belongs exclusive- 



ly to the Mammalia, has in some cases 

 one turn more than in man. 



Whales have an organ of hearing, but 

 the parts are very small. 



Birds have no external ear; only a 

 single ossiculum auditus ; and a short, 

 obtuse, hollow, bony process, instead of 

 cochlea. 



Reptiles have membranous semicircular 

 canals and vestibulum ; generally a single 

 ossiculum audilus, resembling that of 

 birds ; and in some Instances a tympanum, 

 and membrana tympani, level with the 

 surface of the body. 



Fishes hav-a a membranous vestibulum 

 and canals, but no external organs. 



THE EYE. 



A sensibility to the impressions of 

 light is common to all those animals, 

 which in a natural state are exposed to 

 this element ; it appears at least very 

 evidently to exist in some of the most sim- 

 ple zoophytes, as the armed polypes (hy- 

 dra :) but the power of perceiving the 

 images of external objects is confined to 

 those who are provided with eyes for 

 their reception. Nature has bestowed 

 on some species, even of red-blooded 

 animals, a kind of rudiment of eyes, which 

 have not the power of perceiving light ; 

 as if in compliance with some general 

 model for the bodily structure of such 

 animals. This circumstance at least has 

 been asserted of the blind rat (marmota 

 typhlus) among mammalia ; and of the 

 myxine glutinosa among fishes. 



The conjunctiva covering the front of 

 the eye-ball, in the former animal, is co- 

 vered with hair, so that the eye, which 

 is exceedingly small, seems to be com- 

 pletely useless. 



Large animals have small eye-balls in 

 proportion to their size : this is very re- 

 markably the case with the whales. Those 

 which are much under ground have the 

 globe also very small ; as the mole and 

 shrew : in the former of these instances 

 its existence has been altogether de- 

 nied ; and it it is not in fact larger than a 

 pin's head. 



The eyes of man and the simiac are di- 

 rected forwards : in the latter animals, 

 indeed, they are placed nearer to each 

 other than in the human subject. The 

 lemul tarsius has them more closely ap- 

 proximated than any other animal. All 

 other Mammalia have these organs se- 

 parated by a considerable interval, and 

 directed laterally. The same circum- 

 stance obtains in birds, -with the excep- 



