nui 



m:\RT. 



Ut U formed over the vascular area being 

 esMtMd wtU. "blood by UM i-im< " of pur of arterial trunks, 

 SClthc Uood k eofisrtsd from then, by the circular veooos .in,.. 

 wlbeh bow*s ih. area, and i. r*urned to tb. embryo by the venous 



I. UM UU.I..M.I. which are flt oU.rre.1 in the body of 

 a* well aa to the vascular area, no difference U at first 



. etwecB UM character, of the arteries and those of the 



vetaa, Md (km* are only to be dMtJBgniahed by the direction of the 

 a of blood drcokUag through them. 



at abort the fourth or fifth day of incubation the coaU of the 

 begto to appear thicker than those of the veins, and the 

 TWw*7tLeaa soon becomes evident. After the principal 

 , the development of new once appears to take place 

 "Itof as they are to occupy the interspaces existing 

 Jy formed, or are to extend themselves into out- 



f: 



parta. U UM first of UMSS oases the new capillaries appear to 

 rd. Uk* UM 



original once, from stellate cells [CAPILLARIES], 

 meet the vessel* in which the blood is already tirou- 

 feting, coalesce jrith them, and thu. receive the current into their own 

 cantM*. to transmit U to aome other veeeeL But in the second, the 

 Bew vsasiU are formed entirely by extension from those already 

 This takes place to the following mode : Suppose a line, 



. 



or .reh, of capillary veaael. paning below the edge, or surface, of 

 art to which new material baa been tuperadded ; the veaeel will at 

 nntpneent a alight dilatation in one, and ooincidently, or ahortly after, 

 to aooUMr point, aa if Ha walla yielded a little near the edge or 

 The alight ponchea thoa formed gradually extend, aa blind 

 . or verUcula. from the original vessels still directing their 

 i toward* UM edge or aurface of the new material, and crowded 

 with blood-corpoecles, which are punned into them from the main 

 etnam. Still extending, they converge, and meet ; the partition wall 

 that to at Brat formed by the meeting of their cloned end. clean 

 away, and a perfect arched tube ia formed, through which the blood, 

 diverging from UM main or former stream, and then rejoining it, may 

 be coatinuoiuly propelled. Thi. but procea* may be aeen in the 

 ; part* of the tail of the tadpole, in the development of the 

 till, and leg* of the water-newt, in the first evolution of 

 extremities of the ambryoea of higher animals, and in the 

 of new itructuret in the fully-developed organim, either 

 far UM rrpajr of injuries or aa UM remit of morbid processes. In 

 OOM lines n 1 111 it would appear that the wall of the newly-forming 

 veasel give* way, and that UM blood-corpuscles escape from it into 

 MM parenchyma, at first collecting in an undefined mass, but noon 



a definite direction, and coming into connection with 

 i of the arch, or with some adjacent vessel Thus, 

 , and not a vernal, is formed ; and it is probably in 

 this way that those paaeagea are excavated, which take the place of 

 ***fa-* vessel* in many of the lower tribe, of animals, and also, 

 according to Mr. Paget, to some of the softer and (east organised 

 growths in man. 



"The first rudiment of the heart appears about the 27th hour, and 

 ia a mesa of cells, of which the innermost soon break down, so as to- 

 tem a tabular cavity ; for aome time it is simple and undivided, 

 silsoilim however through nearly the whole length of the embryo ; 

 bot the posterior part may be regarded as corresponding with the 

 future auricle, since prolongation, may be perceived extending from 

 that part into the transparent area, which indicate the place 

 where the veto* subsequently enter. Although the development has 

 inmsjeded thus br at about the 35th hour, no motion of fluid is seen 

 to the heart or veesrb until the 38th or 40th hour. When the heart, 

 which may be oooaidared aa analogous at this period to the dorsal 

 veaaet of the Awltda, first begins to pulsate, it contains only colour- 

 lea laid mixed with a few globules. A movement of the dark blood 

 to the ctrcumferenoe of the vascular area is at the same time 

 pemelved ; bat this b independent of the contractions of the heart, 

 ad ft ia not until a subsequent period that such a communication 

 a iiUHlmil between the heart and the distant vessel*, that the dark 

 si mrtatoH to them arrives at the central cavity, and i* propelled 

 by Us tmhtUooa This fact, which we have just seen to poetess a 

 very imporUat bearing on UM theory of the circulation, and which 

 ka.bee.deaM ' 



by aome observer*, appear* to have been positively 

 !!!! I by UM latest research** of TtTvon Baer. 

 'The cnomeUon of this dorsal vessel (for so it may be termed) 

 a* to tke Jswcfeb, at ite posterior extremity, and gradually 

 ft*** to the anterior ; but between the 40th and 60th hours 

 i to He part* may be observed, which i* effected by a con- 

 <md UM middle of the tube; and the dilatation of the 

 I"*"**" portion become* an auricular sac, and that of the anterior a 

 vwrtnenUr cavity. Between the 60th and 0th hours the circulation 

 of tke bleed to the vascular area becomes more vigorous, and the 

 is BO longer continuous with that of the 

 icceed it at a separate period. At the same 

 of the heart become* more and more bent together 

 w become* much shorter 

 of the body, and i* more confined to tho 

 Jteh it u anbsequenUy restricted. The 

 side of the cvrv. which the tube present* is that which 

 Uy become* tk* apex or point of UM heart, and between 



tap* the 



the 0th and 70th hours this U seen to project forward from the 

 breast of the embryo, much in the situation it subsequently occupies. 

 Al. .ut the same time the texture of the auricle differs considerably 

 from that of the ventricle, the auricle containing th tliin and mem- 

 branous walla which it at first possessed ; while the ventricle baa 

 become stronger and thicker, both iu internal and external surfaces 

 being marked by the interlacement of muscular fibres, aa in the 

 higher MoUtuca. About the 05th hour the grade of development of 

 the heart may be regarded as corresponding with that of the fish, the 

 auricle and ventricle being quite distinct, but their cavities are as yet 

 quite single. The heart of the dog at the 21st day bears a great 

 resemblance to that of the chick at the 65th or 60th hour; it consist* 

 of a membranous tube twisted on itself, and partially divided into 

 two principal cavities, besides the bulb or dilatation which at this 

 period is found at the commencement of the aorta, and which 

 corresponds with the bulbus arterioeus of fiithes. 



" Having thus traced the evolution of the heart of the chick up to 

 the grade which it presents in fishes, we may now inquire what is the 

 condition of the other parts of the vascular system at the same time. 

 At the end of the second day the primitive arterial trunk is seen to 

 have divided into two canals, which eeparate from one another to 

 inclose the pharynx, and then unite again to form the aortic trunk, 

 which passes down the spine. During the first half hour of the thirl 

 day a second pair of arches is forme.!, wlii'-h encompagaea the pharynx 

 in the same manner ; and towards the end of the third day twu 

 pairs of vascular* arches are formed, BO that the pharynx ia now encom- 

 passed by four pairs of vessels, which unite again to supply the general 

 circulation. These evidently correspond with the branchial arteries 

 of fishes, although no respiratory apparatus ia connected with 

 and in fact the distribution of the vascular system of the bird on the 

 fourth and fifth days exactly resembles that prc*<nt.'il i>y many 

 cartilaginous fishes, as well as by the tadpoles of the Rntr<" i". Tin 1 

 first pair of arches is obliterated about the cud of the fourth day, but 

 a pair of vessels which is sent from it to the head and neighbouring 

 parts, and which afterwards remains as the carotid arteries, continues 

 to be supplied through a communicating vessel from the second arch. 

 While the first pair is being obliterated a fifth U formed behind the four 

 which had previously existed, and proceeds in the same manner as 

 the fourth from the ascending to the descending aorta. On the fourth 

 day the second arch also becomes less, and on the fifth day ia wholly 

 obliterated, whilst the third and fourth become stronger. From tho 

 third arch, now the most anterior of those remaining, the arteries 

 are given off which supply the upper extremities ; and the vessels of 

 the head are now brought into connection with it by means of the 

 communicating branches, which previously joined the third with the 

 second arch. When these vessels are fully developed, the branches 

 by which these arches formerly sent their blood into the aorta shrink 

 and gradually disappear ; so that about the thirteenth or fourteenth 

 day the whole of the blood sent through the two anterior arches U 

 carried to the head and upper extremities, instead of being transmitted 

 to the descending aorta as before. There now only remain the fourth 

 and fifth pair of branchial arches, the development of which into tho 

 aorta and pulmonary arteries will be described in connexion with 

 the changes which are at the same time going on in the heart. 1 )nrin^ 

 the fourth day the cavities of the heart begin to be divided for :!,. 

 separation of the right and left auricles and ventricles. About the 

 80th hour the commencement of the division of the auricle in indi- 

 cated externally by the appearance of a dark line on the upper part 

 of iu wall, and this after a few hours is perceived to be due to a 

 contraction which, increasing downwards across the cavity, divides 

 it into two nearly spherical sacs. Of these the right is at first much 

 the larger, and receives the great systemic veins; the left has then 

 the aspect of a mere appendage to tho right, but it Hubsequently 

 receives the veins from the lungs when these organs are developed, 

 and attains an increased size. The septum between the auricles is by 

 no means completed at once : a large aperture (which subsequently 

 becomes the foramen orale) exists for some time at its lower part, so 

 that the ventricle continues to communicate freely with both auricles. 

 This passage is often closed by the prolongation of a valvular fold, 

 which meets it in the opposite direction ; it remains pervious ho 

 until the animal begins to respire by the lungs, and sometimes in not 

 completely obliterated even then. The division of the ventricle com- 

 mences some time before that of the auricle, and is effected by a sort 

 of duplicature of its wall, forming a fissure on its exterior and | in- 

 jection on iu interior; and thus a septum is gradually developed 

 within the cavity, which progressively acquires firmness, and rises 

 higher up, until it reaches the entrance to the bulb of the aorta, 

 where some communication exist* for a day or two longer. At lost 

 however the division is complete, and tho inter-ventricular H 

 becomes continuous with the inter-auricular, so that the heart 

 may be regarded as completely a double organ. The progressive 

 stages presented in the development of this septum are evidently 

 analogous to its permanent conditions in tho various species of 

 reptiles ; but it must not be lost sight of that in all reptiles the inter- 

 auricular septum is first developed, and that it is completely i 

 in many instances in which thu inter-ventricular septum is abi" 

 i ni] <! fret. The changes which occur in the heart m '!" l/./mmoiia 

 are of n precisely similar character, and an they take place more slowly 



