CHAP. XXVIII.] OF THE HUMAN HEART. 331 



Of the Heart. This hollow muscular organ, which, like a 

 forcing pump, drives the blood throughout the vascular system, 

 varies in its constitution, according to the complexity of the circu- 

 lation, from a simple muscular tube, such as the dorsal vessel of 

 insects, to the complex double heart of man, with its four cavities, 

 and its beautiful apparatus of valves. 



The dorsal vessel of insects is the most simple condition of the heart. It 

 consists of a muscular tube provided with certain valves, disposed like those 

 of veins ; these, by affording an obstacle to the flow of the blood in one direc- 

 tion, determine the course in which it is propelled by the contraction of the 

 muscular wall, namely, towards the head. It is situated along the middle of the 

 back, whence its name. At the points which correspond to the situation of the 

 internal valves, it exhibits distinct constrictions, which in some insects are so 

 marked, that the vessel appears to consist of " a series of slightly conical seg- 

 ments, partially sheathed one upon the other." (Owen.) The blood is propelled 

 to the head through a tubular prolongation of the dorsal vessel, which corres- 

 ponds to the aorta; this divides into numerous branches, which soon lose them- 

 selves in the areolse, or diffused sinuses, which occupy the spaces between the 

 tissues of the insect ; from these sinuses, as from veins, the blood is returned 

 to the heart, and enters that tube at several points, at its posterior, or caudal 

 extremity, as well as at several apertures which are found at each side of the 

 dorsal vessel, near the points of attachment of the valves. 



In Crustacea, the heart is likewise of a very simple form. In some of 1 he lower 

 Crustacea, it is simply a muscular vessel, as in insects ; in the higher animals of 

 this class, as in crabs and lobsters, it forms a distinct muscular cavity, or ventri- 

 cle, giving origin to arteries, and pierced by several venous orifices through which 

 the blood is poured from the large venous sinuses, which receive it on its return 

 from the body. It is situated, as in insects, beneath the enlargement of the back. 



In the molluscous classes, the heart still retains great simplicity of struc- 

 ture. In the lowest of these, as Tunicata, it is still a muscular vessel, pro- 

 pelling the blood through arteries which ramify on the respiratory organ, 

 whence it is taken up by veins, and returned to the heart. In the compound 

 Ascidians, we meet with the remarkable phenomenon of the oscillation of the 

 currents of the circulation, under the influence of a change in the direction of 

 the peristaltic contractions of the heart. 



In the Acephalous mollusks, we first observe the subdivision of the heart 

 into two compartments or cavities ; an auricle which receives the blood from 

 the veins, and transmits it to a fusiform ventricle, which drives it to the various 

 parts of the body. In one of the most highly organised of the Acephalous mol- 

 lusks, Venus chione, Professor Owen describes two auricles, which receive the 

 blood from the veins of the gills, and transmit it to the single fusiform ven- 

 tricle, which is perforated by the rectum : and in the genus Area, the ventricle 



his Disquisitio Anat -Phys. Organismi Corporis Hum. ejusque Processus Vitalis. 

 Vienna, 1812. Bichat, indeed (1801), had erected these vessels into a system 

 intermediate to that of the arteries, and of the veins ; but no anatomist, who 

 compares the descriptions of the two writers, will hesitate to give to the former 

 the merit of a more intimate practical knowledge of the anatomy of these vessels. 



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