VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 315 



Though the body and tail is composed of a series of bones connected together 

 and moved as in fish, yet it has its movements produced by long muscles, with 

 long tendons, which renders the body thicker, while the tail at its stem is smaller 

 than that of any other swimmer, whose principal motion is the same. Why this 

 mode of applying the moving powers should not have been used in fish, is pro- 

 bably not so easily answered ; but in fish the muscles of the body are of nearly 

 the same length as the vertebrae. 



The depressor muscles of the tail, which are similar in situation to the 

 psoae, make 1 very large ridges on the lower part of the cavity of the belly, rising 

 much higher than the spine, and the lower part of the aorta passes between them. 

 These 2 large muscles, instead of being inserted into 2 extremities as in the 

 quadruped, go to the tail, which may be considered in this order of animals as 

 the 2 posterior extremities united into one. Their muscles, a very short time 

 after death, lose their fibrous structure, become as uniform in texture as clay or 

 dough, and even softer. This change is not from putrefaction, as they continue 

 to be free from any offensive smell, and is most remarkable in the psoae muscles, 

 and those of the back. 



Of the construction of the tail. — The mode in which the tail is constructed 

 is perhaps as beautiful, as to the mechanism, as any part of the animal. It is 

 wholly composed of 3 layers of tendinous fibres, covered by the common cutis 

 and cuticle : 2 of these layers are external, the other internal. The direction of 

 the fibres of the external layers is the same as in the tail, forming a stratum 

 about i of an inch thick ; but varying, in this respect, as the tail is thicker or 

 thinner. The middle layer is composed entirely of tendinous fibres, passing 

 directly across, between the 2 external ones above described, their length being 

 in proportion to the thickness of the tail ; a structure which gives amazing 

 strength to this part. The substance of the tail is so firm and compact, that the 

 vessels retain their dilated state, even when cut across ; and this section consists 

 of a large vessel surrounded by as many small ones as can come in contact with 

 its external surface ; which of these are arteries, and which veins, I do not 

 know. The fins are merely covered with a strong condensed adipose mem- 

 brane. 



Of the fat. — The fat of this order of animals, except the spermaceti, is what 

 we generally term oil. It does not coagulate in our atmosphere, and is probably 

 the most fluid of animal fats; but the fat of every different order of animals has 

 not a peculiar degree of solidity, some having it in the same state, as the horse 

 and bird. What I believe approaches nearest to spermaceti, is the fat of rumi- 

 nating animals, called tallow. The fat is differently situated in different orders 

 of animals; probably for particular purposes, at least in some we can assign a 

 final intention. In the animals which are the subject of the present paper it is 



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