INCUBATION. 



tjon for both thefe accounts, according to our obfervations : 

 tlius we have fometimes fccn with a lens the appearance de- 

 fcribed by Malpiglii, and at the fame time the fingle trunk 

 nearly traiifpareut from wanting red blood, and enveloped 

 and concea ed by the intricate ; iid clofe plexus formed by 

 the extremities of the veins of the external part of the area. 

 We believe, however, that there is in every cafe originally 

 a fmgle veffel which makes the outline of the veinous figure 

 or area, and which exifts previosifly to the vefTels which com- 

 municate with the chick. It is not uncommon to find this 

 veffel deficien: at fome parts of the circumference of the area, 

 more cfpecial y at the fuperior part, where the outline makes 

 the pointed projection inwards towards the hei-d of the chick. 

 Sometimes a vein is feen to pafs acrofs the chink formed by 

 the outhne at this part. 



The arteries which go to the vafcular area bear no propor- 

 tion to the veins. They are fo fmall that they are not eafily 

 feen, and even have been by fome fuppofed not to exill. 

 The left umbilical artery is in general much more coniiuer- 

 able than the right ; indeed we have fometimes doubted 

 whether the latter cxiited or not. They accompany the 

 veins in their courfe, but are not fuppofed to have any in?.me- 

 diate conneftion with the veffe! winch forms the contour of 

 the area. Befides the fmail lize of the arteries at this period 

 of incubation, they are difficult to difcover on account of all 

 the blood-veffels of the egg caiTying blood of the fame co- 

 lour. During the firfl: days the blood is a bright fcariet, 

 or pofTeffes an arterial charafter. The parts compofing the 

 heart of the chick, although formed feveral hours before, 

 are now particularly evident. The vein, which in the firll 

 periods conveyed the blood from the vafcular area to the 

 horfe-ftioe dilatation correfponding to the heart, is now feen 

 to be the vena cava, or rather it {hould be coniidered as the 

 vena port;e and cava together. At the fuperior part of this 

 vein a dilatation is formed, which is prolonged tranfverfely 

 forwards, and inclines a little downwards. This dilated part, 

 in which the vena cava ends, is the origin of the fingle auri- 

 cle of the heart of the fa;tus, and is the firll veficle in order 

 which is feen to pulfate. 



Towards the end of the third day, according to Haller, 

 the enlargement of the head of the vena cava becomes greater, 

 and is diltinguiflied from the reft of the vein by a whitilh- 

 coloured interval ; it is llill, however, but a fingle cavity : 

 its greateft diameter is acrofs from the right to the left fide 

 of the fcetus. The canal which conveys the blood from tlie 

 auricle into the ventricle {le cnnd aurkulmre of Haller), 

 exiils, as already mentioned, before the vena cava becomes 

 dilated to form the auricle, at which time it might be con- 

 fidered as the continuation of the cava. This canal is, there- 

 fore, one of the firil formed parts of the heart. It is moll 

 long and Header when it firil appears, and gradually con- 

 tracts in length, until it is loll in the fubilance of the heart. 

 It is fmallelt where it arifLS from the auricle, and wided 

 where it opens into the ventricle : its diameter, however, is 

 always lefs than that of the latter. It enters the ventricle 

 on the left fide, for which purpofe it defcends a little, and 

 therefore erodes the aorta, which arifes from the ventricle 

 en the right fide and afcends backwards. There arifes from 

 the dcculfation of thefe two vefTcls the appearance of a kind 

 of knot, and the auricular canal feems to double upon itfelf, 

 in order to be continued with the aorta in the oppofite and 

 concealed fide of the knot. 



When the auricular canal is viewed on the right fide, its 

 extremity is concealed, and the manner in which it opens 

 into the ventricle cannot be perceived : it feems rather as if 

 fe^aratedfro.Ti the ventricle by a line which is the border of 



the heart. But if it be- viewed on the left fide, there is na 

 difficulty in difcovering its communication with the ven- 

 tricle. 



The ■ventricle is a fingle cavity during the firft four or five 

 days of incubation ; or rather, the left ventricle exifls only at 

 that period. When the two ventricles make their appear- 

 ance, it is in confequence of the right ventricle being added 

 to that which had already been formed. The left, or fingle 

 ventricle, arifes out of the ring-fhapcd veffel, which con- 

 ftitutes the firft appearance of the heart, and probably is the 

 firft point which pulfatcs. It is an oval veficle, with tranf- 

 pareiit parietes in the commencement ; but it foon acquires- 

 more of a conic figure, with a pointed apex, and greater 

 thicknefs and mufcular power : it is, however, very nearly 

 oval at the prefent period. 



The aorla, as already mentioned, is coeval with the firft 

 appearance of the heart, and exijls before the formation of 

 the auric'.e. It is then very long, and its arch is mucii 

 greater in proportion to the length than in the fuccecding 

 periods of incubation, as in the adult bird. It miikes an 

 extenfive curve backwards, in order to enter the cheft, im- 

 mediately under the head, and compofes a confiderable part 

 of the ring winch reprefents the heart. On the left fide 

 this artery appears as if cut off" by a line which terminates 

 the heart. The auricle, when formed, is furrounded bj- 

 the arch of the aorta, which is fuperior, and more for- 

 wards. 



A few hours after, the part of the aorta which appears to 

 be out fide the cheft, becomes fhorter, and fome inequa- 

 lity is perceived in its diameter. The pnrt of the aorta, 

 which immediately arifes from the ventricle, is more con- 

 tracted than any other ; and the wave of blood which is 

 feen to pafs through it is very (lender. Haller gives the 

 name oi\hs J!i\vt [leJdroit) to this part. 



The next pai-t of the aorta is a dilatation, to which Haller 

 has given the name of the luib of the aorta. This portion 

 of the velTel, as already mentioned, conftitutcs one of the 

 pulfating points, and is iecn to beat even before the auricle. 

 The bulb of the aorta has been milhikcn by Malpighi for 

 the left ventricle, which, confidering its lively and early pul- 

 fation, was not unnatural. He likewife was led by the 

 fame error to take the fingle ventricle, which is atlually the 

 left for the right, or pulmonary ventricle of the heart. 



The third portion of the aorta is cylindrical ; its direc- 

 tion is changed, and it repretnts with the preceding a point 

 or beak. It is fmaller than the bulb, but larger than the 

 branches which go to produce the dorfal aorta. Haller calls 

 this part the bee de I'aorte. 



Soon after the prefent period, and before the end of 

 the fourth dsy, there are branches feen to arife from the 

 beak of the aorta, in order to form the dorfal portion of that 

 veffel. 



The three parts -ivhich compofe the heart of the foetal 

 chick pulfate fo rapidly, when examined immediately after 

 the egg is opened, that the eye cannot follow them; but in 

 proportion as the chick becomes cold and feeble, the inter- 

 vals between the pulfations become longer. Th.e pulfations 

 may be thus gradually reduced to fifteen, to ten, to five, i;r 

 two, or at laft even to but one in a minute ; neverthelefs, th* 

 celerity with which each contraftion is performed is fcarcely 

 diminifhed. 



When the pulfations of the heart are rendered fufficiently 

 flow to be counted, they are perceived to follow each other 

 in a regular fucceffion. The auricle always beats the firft of 

 the three veficles. In its lyftole the point defcends, and the 

 fides are approximated : it fcarcely ever appears to expei 

 J cutitelj- 



