2 INTRODUCTION. 



animals. In the determination of these general principles, or laws of morphology, 

 it is necessary to combine the knowledge of the anatomy and development of 

 animals with that of man. 



PLAN OF ORGANIZATION. 



Vertebrate type. The general plan of construction of the human body agrees 

 closely with that which prevails in a certain number of animals, viz., mammals, 

 birds, reptiles, amphibia, and fishes, and is known as the vertebrate type of organi- 

 zation. The main feature of that type, and that from which its name is derived, 

 belongs to the internal skeleton, and consists in the existence of a median longi- 

 tudinal column, which extends through the whole trunk, and is composed in the fully 

 developed state of a series of bones termed vertebra. This vertebral column is formed 

 in the early embryo around a simple rod-like structure, the primitive skeletal axis, 

 which is called the notochord, and which in most vertebrate animals disappears to a 

 greater or less extent in the course of development. The more solid portions of the 

 vertebrae immediately surrounding the notochord are known as the bodies or centra 

 (figs. 2 and 3), and constitute a pillar around which the other parts are grouped 

 with a certain regularity of structure. At one extremity of this pillar is situated the 

 head, showing in almost all the animals formed upon this type a greater development of 

 its constituent parts ; and at the other the tail in which an opposite character or that 

 of diminution prevails ; while on the sides of the main part or trunfc, there project. 

 in relation with some of the vertebral elements, two pairs of symmetrical limbs. 



The head and trunk contain the organs or viscera most important to life, such as 

 the alimentary canal and the great central organs of the vascular and nervous 

 systems, while the limbs, from which such principal organs are absent, are very 

 variable and differ widely in the degree of their development among the various 

 animals formed upon the vertebrate type. In man and the higher animals the trunk 

 is divisible into neck, chest, abdomen, and pelvis. 



The vertebrate form of skeleton is invariably accompanied by a determinate and 

 conformable disposition of the other most important organs of the body, viz. : 

 firstly, the existence on the dorsal aspect of the vertebral axis of an elongated cavity 

 or canal which contains the brain and spinal cord, or central organs of the nervous 

 system ; and secondly, the existence on the ventral aspect of the vertebral axis of a 

 larger cavity, the visceral cavity, body cavity or co&lom, in which are contained the 

 principal viscera connected with nutrition and reproduction, such as the alimentary 

 canal, the heart and lungs, the great blood-vessels, and the urinary and generative 

 organs. 



The general disposition of the parts of the body and of the more important 

 viscera in their relation to the vertebral axis are shown in the accompanying 

 diagrams of the external form and longitudinal and transverse sections of the human 

 embryo at an early period of its existence. 



Segmentation of the body. The vertebrate type of organisation in the repetition of 

 similar structural elements in a longitudinal series, has a segmented character, especially in 

 the axial portion of the body, and this segmentation affects more or less, not merely the 

 skeletal parts of its structure, but also, to some extent, its other component organs. 



A segmented plan of construction is by no means restricted to vertebrate animals, but exists 

 in several other classes of the animal kingdom, as is most conspicuously seen in the Arthropoda, 

 such as insects and Crustacea, and in the Annelida or worms. These animals, however, although 

 showing a serial repetition of parts of like structure, are not considered to belong to the verte- 

 brate type of organization. 



In the human embryo, as in that of all vertebrate animals, the segmentation is most marked 

 in the muscular system, the nervous and osseous systems becoming for the most part corres- 

 pondingly marked off : in the adult the osseous and nervous systems retain in great measure 

 the segmentation which has thus been produced, although in the muscular system it has 



