DEVELOPMENT OF BLOOD, &c., IN MAN AND MAMMALS 409 



a more or less transverse direction. Two plates of illustrations of the left ventricle of the heart are given further on 

 (Plates xcvii. to xcviii.)- It may be stated in passing that the muscular fibres of the stomach, bladder, and uterus are 

 distributed in much the same fashion. These are illustrated by four plates (Plates c. to ciii.). The left ventricle 

 of the heart, which is the chief propelhng organ in the circulation of the blood, is symmetrical, and constructed on 

 mathematical principles to secure the greatest possible strength with the least possible material. It is stayed and 

 strutted in every direction, after the manner of a girder bridge, to resist strain at every point. It is expressly 

 designed and expressly built to withstand continuous wear and tear, such as no other organ in the body is exposed 

 to. The heart forces the blood through the system during the entire life of the individual. It rests neither day 

 nor night from the cradle to the grave, and on its efficiency the health and hfe of every one of us depends. It 

 forms the central engine of all our activities, and any imperfection in it inevitably results in disaster. It is no 

 wonder that so much prevision and care should be expended in its production. Whatever may be said of other 

 animal organs, the heart (and particularly the left ventricle) is no chance product. Its structure and function have 

 only to be examined to convince every one who reflects, that it is a fundamental organ, in large measure necessary 

 to, and coeval with, life ; an organ, the foundations of which are laid in the foetus and perfected soon after birth. 

 It is the great prime mover of the blood and muscles from the physical point, as the brain is the great prime mover 

 from the mental or psychical point. In order to carry on its important work it is endowed with a centripetal and 

 centrifugal power ; each compartment having a double independent movement, whereby it alternately opens, expands, 

 and sucks blood into it, and closes, contracts, and forces blood out of it. Each compartment exercises a vis a 

 fronte and a vis a tergo power ; the heart forming one of the most perfect and effective forcing-pumps known to 

 modern science. 



§ 83. The Development of Blood, Blood-vessels, Nerves, Muscles, Bone, Lungs, Glands, Sense Organs, 

 &c., in Man and Mammals. 



The manner in which the red and white blood-corpuscles, blood-plasma, blood-vessels, nerves, muscles, bone, 

 glands, sense organs, &c., are produced in the embryo is exceedingly interesting from a teleological point of view. 

 These several substances and structures, various as regards composition, consistence, and ultimate function, are all 

 being developed at, or nearly at, the same time. The one is not manufactured out of the other. On the contrary, 

 they spring from separate centres, and each grows in its own particular way and independently. They do not 

 crowd or jostle each other, but fit into their own particular niches in the organic edifice as specially-ordered, dressed 

 stones fit into a carefully-planned building. There is no waste either of energy or material, and the completed 

 animal bears the unmistakable impress of an intelhgent Designer and Constructor. The production of the several 

 structures referred to is spontaneoiis, and in no sense due to irritability, extraneous stimulation, or environment. 

 There is division of labour from the first. Each substance is supreme in its own department. The organism 

 does not consist of a vicarious arrangement of substances and of parts. On the contrary, everything is independent 

 and, within limits, interdependent, in the sense that each thing forms a part of the same whole. The structures 

 be they simple or complex, are typical of their kind, just as the several varieties of plants and animals are typical. 

 To take examples : the young blood is formed simultaneously with the blood-vessels which are to contain it ; the 

 brain, spinal cord, and nerves are growing at the same time as the muscles they are to control ; and the bones are 

 developing fari fassu with the muscles to which they are to afford attachments and leverage, and by which they 

 are to be moved. There is no waiting of any one part of the embryo on any other part ; all are up to time, and 

 everything moves with the regularity of clockwork, and precisely as a planned and ordered whole would be expected 

 to move. " The first red blood -corpuscles are formed very early in embryonic hfe simultaneously with and in the 

 interior of the first blood-vessels. They are developed in the mesoblast in a circular area which surrounds the part 

 of the blastoderm which is occupied by the developing body of the embryo. The area is known as the vascular area, 

 and the first blood-vessels and blood-corpuscles are, therefore, formed outside the actual body of the embryo. Those 

 mesoblastic cells in the vascular area which are concerned with the formation of vessels (angioblasts) become extended 

 into processes of varying length, which grow out from the cells in two or more directions. The cells become united 

 with one another, either directly or by the junction of their processes, so that an irregular network of protoplasmic 

 nucleated corpuscles is thus formed. Meanwhile the nuclei become multiphed, and whilst the greater number 

 remain grouped together in the original cell-bodies or nodes of the network, some are seen in the uniting cords. The 

 nuclei which remain in the nodes accumulate, each one around itself, a small amount of cell-protoplasm. The 

 corpuscles thus formed acquire a reddish colour, and the protoplasmic network in which they lie becomes vacuolated 

 and hollowed out into a system of branched canals enclosing fluid in which the nucleated coloured corpuscles 

 float. As soon as the heart is developed, or even before this happens, the blood begins to move within the vessels 

 of the vascular area. These first-formed red blood-corpuscles are nucleated cells resembling the pale corpuscles 

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