532 TYPES OF NUTRITION. 



head ; but the venous blood which is in the right ventricle is driven 

 therefrom through the pulmonary artery and ductus arteriosus into the 

 descending aorta, and, mingling with the arterial blood therein, passes 

 to the trunk and legs. Of this blood a portion is then carried to the 

 placenta to be arterialized. 



At the moment of birth a change takes place in the manner of the cir- 

 culation, which is now arranged upon the type described at page 134. 

 This is accomplished as described at page 148. 



From the description which has thus been given, it may be gathered 

 Three types of ^ at U P to ^ G period of birth, three distinct types of nutri- 

 nutrition. tion have been followed. They may, with sufficient accura- 

 cy, be designated, 1st. Yolk nutrition ; 2d. Tuft nutrition ; 3d. Placental 

 nutrition. To these may be added the two followed at a later period : 

 4th. Lactation, and, after the dental mechanism is supplied, 5th. The diet 

 of mature life. 



Respecting the development of special organs, it may be remarked that 

 The vertebral f those which are permanent, the vertebral column is one of 

 column. the first to appear ; it shows itself under the form of isolated 

 quadrangular elements. The gelatinous cellular structure, chorda dorsa- 

 lis, acquires a sheath, which assumes a fibrous structure, and from this, 

 in the lower vertebrates, the vertebrae are evolved. In man, the ele- 

 mentary quadrangular plates are considered tcfhave an independent ori- 

 gin. As they increase in number and size they surround the chorda, 

 and projections springing from their superior surface form arches to en- 

 velop the spinal cord. Each vertebra, therefore, is constructed by the 

 union of two pieces, one on either side. These first assume the condi- 

 tion of cartilage, and, later, the body and arches ossify from separate 

 points. The chorda dorsalis, which has, during this development, been 

 gradually evolved in the bodies of the vertebrae, disappears. 



The bones of the skull are metamorphosed vertebras, of which, accord- 

 ing to Professor Owen, four appear to have undergone change. To these 

 the auditory, gustative, optic, and olfactory nerves are respectively re- 

 lated, in the same manner that the spinal nerves are to their vertebra. 



In the descriptions given in the preceding part of this work, incident- 

 Development al allusion to a sufficient extent has been made to the devel- 

 tus^f organic P ment ^ most of the apparatus of organic and also animal 

 life. life. It may therefore here be briefly stated that the ali- 



mentary canal originates in the pinching off of a part of the blastodermic 

 vesicle below the spinal column. At first it is a straight tube, which 

 communicates about its middle with that vesicle, but after a time shows 

 its eventual division into oesophagus, stomach, large and small intestines, 

 assuming an oblique position on the part to be occupied by the stomach, 

 and then curving in the region of the intestine. From a part of this tube 



