VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 603 



nerves, presented the common appearances. The size of the heads, and the in- 

 terior capacity of the skulls, both in the bradypus tridactylus and the lemur tardi- 

 gradus, seemed smaller in proportion than is usual among animals, so that the 

 quantity of brain must be less than ordinary. 



The effect of this peculiar disposition of the arteries, in the limbs of these slow- 

 moving quadrupeds, will be that of retarding the velocity of the blood. It is well 

 known, and has been explained by various writers, that the blood moves quicker 

 in the arteries near the heart, than in the remote branches; and also, that fluids 

 move more rapidly through tubes which branch off suddenly from large trunks, 

 than if they had been propelled for a considerable distance through small-sized 

 cylinders; besides the frequent communications in the cylinders of the bradypus 

 tridactylus must produce eddies, which will retard the progress of the fluid. From 

 these and a variety of other facts, it will appear, that one effect on the animal 

 economy, connected with this arrangement of vessels, must be, that of diminishing 

 the velocity of the blood passing into the muscles of the limbs. It may be difficult 

 to determine, whether the slow movement of the blood sent to these muscles be a 

 subordinate convenience to other primary causes of their slow contraction, or 

 whether it be of itself the immediate and principal cause. The facts at present 

 ascertained, relative to muscular motion, do not authorize me to treat decidedly of 

 the share which the vascular system holds in the operation of muscular contraction. 

 Certain it is, that a larger proportion of arteries is sent to the muscles of qua- 

 drupeds, than to the ordinary substances; and the extreme redness of these organs 

 shows that their capillaries are of large diameter. A greater degree of redness is 

 also observable in those muscles, of the same animal, which are most frequently 

 called into action. The habits of life among the tardigrade animals, give occasion 

 for the long continued contraction of some muscles in their limbs: these creatures 

 are represented clinging to the boughs of trees, and remaining thus, without loco- 

 motion, for several hours. The powers which require so long a time to determine 

 the contraction of a series of muscles, are probably no less slow in restoring the 

 parts to their former condition; or, if the restoration is to be affected by antagonist 

 muscles under the same circumstances, then the flexion and extension of every part 

 of the limbs will correspond, as to time. 



I have not met with any arrangement of blood-vessels analogous to those 

 described, except in the carotid artery of the Lion. May not this peculiarity be 

 subservient to the long continued exertion of the muscles of his jaws, while hold- 

 ing a powerful animal, such as a horse or buffalo, and thus enable him to retain his 

 prey, till it is wearied out by ineffectual struggles? I believe also, that those 

 animals which chew the cud have a plexus of arteries in the neck, analogous to 

 the rete mirabile: but this fact has not yet been verified in all the ruminating 

 quadrupeds; and the effect of these arrangements seems rather to operate as 

 sluices to the arteries of the masticating muscles, than directly as the means ot re- 



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