TLAN OF ORGANISATION. 3 



ing in almost' all the animals formed upon this type the character of 

 increased 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 irunlc, there project in relation 

 with some of the vertebral elements two pairs of symmetrical limljs 

 in two situations, which are determinate and similar in different 

 animals. 



The head and trunk contain the organs or viscera most important to 

 life, such as the alimentaiy canal and the great central organs of the 

 sanguiferous and nervous systems, while the limbs, from which such 

 principal organs are absent, are less constant and differ more in 

 the degree of their development among the various animals formed upon 

 the vertebrate type. The whole body may thus be regarded as being 

 formed of an axial portion consisting of the head and trunk, and of 

 appendicular portions comprising the limbs. In man and the higher 

 animals especially, the trunk presents a division into the neck, chest, 

 abdomen, and pelvis. 



The vertebrate form of skeleton is invariably accompanied by a 

 determinate and comformable disposition of the other most important 

 organs of the body — as Jirsf, by 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 

 second, by the existence on the ventral aspect of the vertebral axis of a 

 larger cavity, the visceral cavity, in which are contained the principal 

 viscera connected with nutrition, such as the alimentary canal, the 

 heart and lungs, the great blood-vessels, and the urinary and genital 

 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 figures of the external form and longitudinal and 

 transverse sections of the human embryo at an early period of its 

 existence, when its structure resembles more closely that of the lower 

 animals. 



Fig. 1. — Lateral View of Fig. 1. 



THE Human Embryo about 

 Seven Weeks old ; the 

 Vertebral Axis placed 

 horizontally. magnified 

 ABOUT 7 Diameters. (A.T.) 



s, s, indications of tlie ver- 

 tebral divisions along the line 

 of the back ; r, u, anterior 

 or upper limb ; t, f, posterior 

 or lower limb ; u, umbilical 

 opening. In the cranial part 

 the divisions of the brain are 

 indicated, together with the 

 eye, and a u, the auditory 

 vesicle ; near b, the ventral 

 plates of the head with the rudiments of the upper and lower jaws. These plates and 

 the apertures between are represented in a state belonging to a somewhat earlier stage 

 than the rest of the body. 



Segmented Character. — The vertebrate type of organisation possesses, there- 

 fore, in the repetition of similar structural elements in a longitudinal series 



