I N C 



the fyilem of the chick. It is already a vital fluid, wKich 

 is proved by Mr. Hunter's experiments upon the tempera- 

 ture of eggs. (Sec Egg.) If the humours of the egg did 

 not podefs vital properties, they would very foon become 

 putrid from the heat of incubatioa. 



Wl'.cn we contemplate tl»c ftruftiire of tlie volk-bag, it 

 mud be perceived to be not only continuous with the iutef- 

 tinal canal, but to refemble the latter in organization, ex- 

 cept tint the mufcular coat is wanting;. The external 

 membrane of the bag differs in no refped from the peritoneal 

 coat of the inteftines ; and the internal membrane is fo 

 analogous to t!>e villous coat, that wc cannot liefitate to al- 

 low them the fame funilions. No advantage, therefore, 

 would be gained by the pailage of the volk into the in- 

 teftine. 



The abforption of this humour, by the valvular or in- 

 ternal membrane of the yolk-bag, feems proved by the 

 organization of the yolk undergoing a chan^'e upon the 

 furface which is in contact with the membrane, while the 

 central part is unaltered. The circumllance alfo, of a con- 

 crete fubdance, rcfembling the refidue of the albumen, being 

 left in the yolk-hag after the nutritious parts of the yolk 

 are confumed, affords a ftrong evidence that the latter are 

 removed by a procefs of abforption ; for if the yolk pafled 

 through the duftus vite'.lo inteilinahs, it would difappear 

 vithout diftindion of parts. 



The authors who believe that the yolk is carried into 

 the fyftem without a previous preparation in the alimentary 

 canal, attribute the office of abforption to the red veins. 

 In proof of this opinion Btumenbach and others ftate that 

 they have detected upon fome occafions a yellow fluid in 

 the veins of the bag, which they have fuppofed to bo the 

 yolk in its paffage. Notwithftanding this appearance, which 

 we have fomctimes alfo fcen, it appears to us moll probable 

 that the yolk is abforbed by the fame fyilem of veifels 

 •which take up the chyle from the fmall inteilines. The 

 ■yolk-bag and inteftinal canal, as already mentioned, are 

 continuous parts, and fimilarly organized. The yolk-bag 

 is, in facl. a procefs of the inteftines, and fliould be con- 

 fidered as a portion of the body of the chick. It is fup- 

 pliedby the fame blood-vefiels as the inteftines ; why (liould 

 we therefore deny it the ladleal veffels ? 



Another argument againft the abforbing function of the 

 veins of the yolk-bag may be deduced irom the fize of 

 thefe veftels at different periods. During the early part 

 of incubation they are large and numerous ; but after t)ie 

 yolk-bag paffes intoihe abdomen of the ciiick, where their 

 abforbing funftion chiefly, if not entirely, exift s, if it does at 

 all, they become fmaller and fewer llian tlie arteries. The 

 veffels of the yolk-bag, when they conilitute the vafcular 

 area, are the organs of fanguification for the chick. After 

 incubation they appear to become the nutritious veffels of 

 the bag itfelf. 



It may feem not eafy to aflign a ufe for the dudus vitello 

 inteftinali?, ilnlefr. we grant it to be for tranfmitting the 

 fubftancc of the yolk into the alimentary canaL If, how- 

 ever, this (hould not be the cafe, ftill a communication 

 would probably exill between the yolk-bag and the in- 

 teftine ; for it is wiihout example, we believe, for the con- 

 tinuity to be interrupted by two adjoining cavities that 

 have' a fimilar organization. The duftus viicUo intellinalis 

 ve think, therefore, would exill as a matter of courfe, 

 independently of any offices it might perform. We have 

 defcnbcd the purpofcs this part fcrvis in the full grown 

 bird ; it is not, therefore, without its ui'es. 

 . Vol. XIX. 



I N C 



For the hiftory of foetal life in oviparous animals in ce- 

 neral, feeOv.M. ^ 



INCUBUS, in Medicine, from the Latin incubare, to 

 lie ot fit ii^on, the fame with the Englilli night-mare, fignifiei 

 a ftate of imperfect lleep, accompanied with an oppreffion 

 of the breathing, and frightful dreams. 



By the Greeks this alTedion was defignated by a name 

 of fimilar import, \Zti\-r,:, ephialles, from the verb 'f(aX\^fj:a.t, 

 I leap upon ; and it has received various otlier appellations 

 expreffive of the fenfe of weight and oppreffion of the 

 chefl, the laborious refpiration, &c. as well as indicative 

 of the hypothcfes by which its occurrence was explained. 

 Thus it has been called l-r^Srj.r, -^nyixi;, epilepfia neffurna, 

 ajlhma noftiirriwi, SiC. ; and by piiny, ludibria fauni, from 

 the fame notion wliicli dictated the more modern appellations 

 oi hag-ritiirig, 'u.'iz.ard-prejfwg, mare-riding ; dier truth r, ot das 

 reilen der tnahr, of the Germans ; cochanar of the French ; 

 night-mare, &c. 



This diforder feizes perfons, while fleeping, who imagine 

 that tliey feel an extraordinary comprefiion or weight about 

 the breall and ftomach, wliich they cannot by any eilbrt 

 fhake off. In this agony tliey figh, groan, and utter in- 

 diftintl founds ; fometimes they cry out, but more fre- 

 quently they attempt to fpeak, or to move in vain. Thefe 

 feelings give rife to various frightful fuggeftions of the 

 imagination : the patient fancies himfelf to be ftruggling 

 with ftrong men or devils, or to be in a houie on fire, or 

 in danger of being drowned in the fca or fome river ; and 

 in attempting to run away from danger, or climb up a hill, 

 he fancies he falls back as much after every flep as he had 

 advanced before. At length the fenfations of oppreffion 

 become intolerable, and the patient awakes ; but the terror 

 excited by the frightful ideas attending the night-mare, 

 leaves often a palpitation of the heart, with great anxiety 

 and languor, and fometimes a tingling of the ears, and a 

 tremor over the whole body. 



It is altogether uuneceffary to attempt an enumeration 

 of the numerous hypothetical explanations which have been 

 attempted to be given of the phenomenon of incubus, and 

 which have been detailed by Aweii, Bond, and other ivrit--rs. 

 (See Awen, Pofit. iiiaugur. de Inrubo. Argentorat. 1676. 

 Ijond, Effay on the Incubus or Night-mare, 1753 ) 1"''^ 

 dilorder has commonly been fuppofed to proceed from a 

 llagnation of the blood in the finufes of the brain, or in the 

 veffels of the lungs', or from too great a quantity of blood 

 being fent to the head. The horizontal pofture, in time of 

 fleep, and the preffure of the ftomach upon the aorta, in a 

 lupine fituation, have been tiiought fufficient to occafion a 

 more than ufual diftention oi x.\v± fmufes and other vefels of 

 the brain ; and the weight of the licart, preffing on the left 

 auricle and large trun.ks of the pulmonary veins, nr.ay, it is 

 fuppofed, prevent the eafy return of the blood from the lungs-, 

 and thus produce an oppreffion and fenfe of weight z^A futfb- 

 cation in thebreaft. (See Bond, chap, ii.) But without entering 

 into a p.irticular examination of thefe opinions, which are far 

 from being fatisfaftory, we may obferve, with Dr. AVhytt, 

 that, if they were true, fome degree of the night-mare ought 

 to happen to every body that lies upon his back, efpecially 

 after eating a full mcaL Further, if a horizontal fituatiou 

 could overcharge the brain with blood, fo as to occaJioii 

 the tiicubus, how comes it that people, who remain for fome 

 time in an inverted pofture, do not feel thisdifeafe beginning 

 to attack them ? And why does a fiighter degree of the 

 night-mare fometimes feizc people who fleep in an eredl 

 fituation in a chair ? a circumftancc which fometimes occurs, 

 not only after eating, but when the ftomach is out of 

 F order, 



