96 OBSTETRICAL PHYSIOLOGY. 



their origin and contracted in the middle. Their free extremity is flat^ 

 and is either simple or divided, according to the foot of the species. In 

 these prolongations the cartilaginous segments are formed, which, when 

 ossified, at a later period, constitute the bones of the limbs. 



The muscles are divided into four groups, after their origin. They are 

 the vertebral muscles^ which arise from the muscular laminae of the proto- 

 vertebrae ; the visceral muscles for the thoraic and abdominal cavities, the 

 neck and jaws, and having the same origin ; the cutaneous muscles, which 

 originate from the cutaneous laminae of the middle layer of the blas- 

 toderm ; and the muscles of the limbs, whose development is not fully 

 understood. 



It was believed at a certain period, that the muscular fibres are formed 

 by the joining together at their ends of several elongated cells. It is 

 now known that each fibre is constituted by a single cell which extends 

 in length, and whose nuclei are multiplied and placed at the surface ; 

 while the contents are transformed into a substance that presents all the 

 characteristics of contractile tissue. The sarcolemma is formed after the 

 fibre, by a modification of the connective tissue surrounding it. 



The locomotory apparatus of the majority of the domesticated species 

 of animals is so developed at birth, that immediately after that event 

 the young creature can move with more or less alacrity. With the 

 larger herbivorous quadrupeds, the limbs of the new-born animal are 

 long to enable it to reach the teat, as well as to enable it to escape by 

 flight should danger be apprehended. 



The Circulatory System. 



The development of the circulatory system is not apparent until some 

 days after the appearance of the embr}^o in the blastodermic layer, there 

 being no trace of vessels in the germinative space. It is not long, how- 

 ever, before the central organ of oirculation and some vessels appear in 

 the middle layer, and canals also spread to the surface of the umbilical 

 vesicle. While the contents of the vitelline vesicle are being imbibed by 

 the embryo, the heart is in course of formation ; blood-vessels are increas- 

 ing and extending, the allantois is completed, and, finally, the placental 

 circulation is established and continues until birth. 



The pleuro-peritoneal cavity in the embryo shows a diverticulum in 

 front — the cardiac cavity, in the inferior of which the heart is developed. 

 This organ is at first a small mass of cells, the innermost . of which 

 separate in order to leave a little space, and to create blood globules. 

 As soon as it appears, the heart contracts and relaxes alternately, the 

 movements being very slow, though they gradually become accelerated. 

 Towards the twelfth day the organ has the shape of a cylindrical con- 

 tractile tube. From its anterior part arise two branches, the aortic 

 arches, which at first proceed towards the head of the embryo, but 

 afterwards are bent backward and downward. These arches join to form 

 the single aorta, which in its turn divides into two branches — the sub- 

 vertebral or common aortas, — which run parallel along the lower surface 

 of the embryo, giving off four or five branches, the omphalo-meseraic 

 arteries, which ramify in the germinative area and end in a vein — the 

 terminal sinus. From this ramification and sinus arise two vessels, the 

 omphalo-meseraic veins, which return to the posterior extremity of the 

 heart. The circulation in the umbilical vesicle is very ephemeral in. 



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