INTRODUCTION. 3 



subsequently lose all trace of such a subdivision. The parts thus repeated are said 

 to be serially homologous. But there are other instances of serial homology besides 

 those which are manifestly produced by segmentation. The upper limb is serially 

 homologous with the lower limb : each is composed of parts which, to a large 

 extent, are repeated in the other, and the correct adjustment of this comparison 

 between the several parts of the upper and lower limbs constitutes one of the most 

 difficult and yet interesting problems of morphology. 



Homoplasy is a term which has been introduced to express a form of corre- 

 spondence between organs in different animals which cannot be included under 

 the term homology. Two animal groups, which originally have sprung from the 

 same stem-form, may independently develop a similar structural character which is 

 altogether absent in the ancestor common to both. Thus the common ancestor of 

 man and the carnivora in , all probability possessed a smooth brain, and yet the 

 human brain and the carnivore brain are both richly convoluted. Not only this, 

 but certain anatomists seek to reconcile the convolutionary pattern of the one with 

 the convolutionary pattern of the other. What correspondence there is does not, 

 in every instance, constitute a case of homology, because there is not in every case 

 a community of origin. Correspondence of this kind is included under the term 

 " homoplasy." Another example is afforded by the heart of the mammal and that 

 of the bird. In both of these groups the ventricular portion of the heart consists 

 of a right and a left chamber, and yet the ventricular septum in the one is not 

 homologous with the corresponding septum in the other, because the common 

 ancestor from which both have sprung possessed a heart with a single ventricular 

 cavity, and the double-chambered condition has been a subsequent and independent 

 development in the two groups. 



Systematic Anatomy. The human body is composed of a combination of 

 several systems of organs, and the several parts of each system not only present a 

 certain similarity in structure, but also fulfil special functions. Thus there are 



1. The skeletal system, composed of the bones and certain cartilaginous and mem- 

 branous parts associated with them, the knowledge of which is known as osteology. 



2. The articulatory system, which includes the joints or articulations, the 

 knowledge of which is termed arthrology. 



3. The muscular system, comprising the muscles, the knowledge of which con- 

 stitutes myology. 



4. The nervous system, in which are included the brain, the spinal medulla, the 

 ganglia of the spinal and cerebral nerves, the sympathetic ganglia, and the various 

 nerves proceeding from and entering these. The knowledge of these parts is ex- 

 pressed by the term neurology. In this system the organs of sense may also be 

 included. 



5. The Hood vascular and lymphatic system, including the heart, blood-vessels, 

 the lymph vessels, and the lymph glands. Angeiology is the term applied to the 

 knowledge of this system. 



6. The respiratory system, in which we place the lungs, windpipe, and larynx. 

 *7. The digestive system, which consists of the alimentary canal and its associated 



glands, and parts such as the tongue, teeth, liver, pancreas, etc. 



8. The urogenital system, composed of the urinary organs and the reproductive 

 organs the latter differing in the two sexes. 



The term splanchnology denotes the knowledge of the organs included in the 

 respiratory, digestive, and urogenital systems. 



9. The integumentary system consists of the skin, nails, hair, etc. The know- 

 ledge of this system is termed dermatology. 



