PHYSIOLOGY. 



Sweat seems to be nothing more than 

 an increase of the insensible perspiration 

 produced by augmented action of the cu- 

 taneous vessels. Increased temperature 

 and exercise give rise to it ; and it fur- 

 nishes the most powerful means of re- 

 ducing that augmented temperature, ac- 

 cording to the well known frigorific 

 effects of evaporation. Hence the human 

 body has borne a greater heat than that 

 of boiling water, without having its own 

 temperature raised. 



Functions of the Brain andJ^erves. The 

 organs of the animal functions, which 

 keep up the connection between the 

 body and the faculties of the mind, and 

 are "therefore found only in animated or- 

 ganized bodies, may be conveniently di- 

 vided into two classes ; the sensorium, 

 and the nerves : the latter including the 

 nerves and their origins from the brain ; 

 the former comprehending the rest of 

 the cerebral organs, by which the offices 

 of the nerves are connected with the 

 more noble part of our frame, the facul- 

 ties of the mind ; and which may there- 

 fore not unaptly be termed the organ of 

 the mind. For the differences which 

 animals present in a comparison of the 

 proportions of these parts, as well as in 

 the size of the brain altogether, &c. see 

 COMPARATIVE ANATOMY. 



The brain, when brought into view by 

 a removal of the cranium, presents a dou- 

 ble motion ; it rises slightly during expi- 

 ration, and subsides again when the 

 thorax is dilated. This is explained from 

 the temporary obstruction which the re- 

 turn of the venous blood experiences 

 when the lungs are compressed. But a 

 more conspicuous elevation and depres- 

 sion of the cerebral surface arises from 

 the impulse of blood into the arteries of 

 the head ; this motion is therefora per- 

 fectly synchronous with the pulse, and 

 may be felt in every infant whose fonta- 

 nelle is not closed. The quantity of blood 

 received by the brain is very considera- 

 ble ; according to Mailer's calculation, be- 

 tween two-thirds and a half of the whole 

 mass of blood that enters the aorta. This 

 blood is circulated through all the minute 

 and numerous arterial ramifications of the 

 pia mater, before it enters the brain, as 

 it should seem, in order to diminish its 

 impulsive force ; it rises contrary to its 

 gravity, and its conducting tubes have an 

 angular and tortuous course before they 

 branch out on the pia mater ; which cir- 

 cumstances augment the retarding effect. 

 Every thing, on the contrary, facilitates 



the blood's return, and prevents venous 

 distension. 



The vast and wonderfully complicated 

 vascular apparatus of the brain, and the 

 large proportion of blood sent to the 

 organ, naturally lead us to expect, that 

 this part and the heart are closely depen- 

 dant on each other. If the cerebral ar- 

 teries be all tied, the animal perishes in- 

 stantly. The influence of the heart, 

 essential to the preservation of life, does 

 not seem to consist so much in the agita- 

 tion which the cerebral arteries commu- 

 nicate to its substance, as in the effect 

 which the arterial or oxygenized blood 

 exerts on it. For if venous blood be sent 

 into the head, instantaneous death en- 

 sues ; and this seems to be the way in 

 which the cessation of respiration, by 

 drowning, hanging, &c. proves fatal. 



Nerves, which arise from the brain and 

 spinal marrow, and are the organs con- 

 veying the impressions of external ob- 

 jects to those parts, are not found in all 

 structures of the body. The cellular sub- 

 stance, cuticle, rete mucosum, hair and 

 nails ; cartilages, bones, teeth, perios- 

 teum, and marrow; tendons, aponeuroses, 

 and ligaments ; membranes, as the dura 

 mater, pleura, pericardium, peritoneum, 

 &c.,the cornea, &.c.,the absorbent system, 

 the secundines and umbilical chord, are 

 in this predicament. For an account of 

 the disputes which have arisen concern- 

 ing the sensibility of these parts, see the 

 introductory remarks on sensibility and 

 contractility. 



The ultimate points of origin of the 

 nerves from the brain, are hitherto hard- 

 ly determined; so that it is still question- 

 ed, whether or no the right and left 

 nerves decussate. That this is the case 

 in the optics is tolerably clear ; and the 

 fact, that injury of one side of the brain 

 causes paralysis of the opposite side of the 

 body, has led to an inference, that the 

 same decussation obtains in all the nerves. 

 The ganglia and plexuses, in which diffe- 

 rent nerves are united together, proba- 

 bly perform an office analogous to the 

 arterial communications; that of pre- 

 serving the connection of any part with 

 the brain, when the direct communica- 

 tion is cut off. 



Physiologists have endeavoured to trace 

 the termination of nerves in the organs 

 which they supply ; but the research is 

 almost too subtle for our imperfect modes 

 of investigation. In some instances, as 

 the optic and auditory, they are manifest- 

 ly resolved into a soft pulp : and we con- 





