TSE BODY PLANES. 33 



amined, tlie degree of development, or the part which, is divided. It lies 

 upon the meson, and serves therefore as a boundary between the right and 

 the left sides of the animal. 



In most animals it is located, in nearly its whole length, nearer that side 

 of the body which is commonly upward in ordinary locomotion ; that is, it is 

 nearer the dorsal aspect. There is no definite plane of contact of the dorsal 

 with the ventral region as of the right with the left side ; but, for convenience, 

 the two regions may be regarded as meeting at a line X, X', passing from side 

 to side through the middle of the Columna vertebralis. 



Dorsad of the Columna is a canal, the Canalis neuralis, containing the 

 myelencephalon or cerebrospinal axis, (brain and spinal cord). Ventrad of 

 the Columna is a cavity, usually more capacious, the coelum, or general body 

 cavity, in which are the heart, alimentary canal, and other viscera. 



This arrangement of principal organs, in two cavities, on opposite sides 

 (but not on the right and left) of a mesal axis is a constant character of aU 

 Vertebrates, and, with perhaps a few exceptions, is peculiar to the group. 



§ 59. Pigure 4 represents an ideal hemisection (section on the meson) of 

 such a body as is represented in figures in 2 and 3. The cephalic aspect or 

 head looks toward the left. 



§ 60. The Body Planes. — For the sake of precision in the use of 

 toponymical terms, the planes already referred to will be more distinctly 

 defined ; see Fig. 5. 



§ 61. 1. The Meson. — This is a plane passing lengthwise of the body, 

 and dividing the whole into approximately equal and similar right and left 

 halves. 



For convenience, the dorsal and ventral bordei's of this plane may be 

 called the dorsimeson and the ventrimeson respectively. 



Organs like the nose, the stomach, and the urocyst (bladder) appearing 

 upon both sides of the meson are said to be mesal or azygous. They are, at 

 least primarily and approximately, symmetrical in themselves ; that is, they 

 consist of similar right and left halves. 



Organs like the eyes, the lungs, and the kidneys, which form pairs, the 

 one upon the right and the other upon the left of the meson, are called 

 lateral or paired antitropous organs. Either of them is symmetrical with 

 its fellow ( platetrope), iut not in itself. 



§ 63. 3. An imaginary transverse, dorso-ventral plane, at right angles 

 with the meson, and dividing the body into a cephalic and a caudal region. 



§ 63. 3. An imaginary longitudinal, dextro-sinistral plane, extending 

 the whole length of the body, and dividing it into a dorsal and a ventral 

 region. 



These three body-planes correspond in direction with the three dimen- 

 sions of a solid, length, height, and width. Each is at a right angle with 

 both the others. The longitudinal somatic axis lies in the meson. 

 3 



