18 INTRODUCTORY LECTURE. 



suspended in a particular compartment of the general cavity of the 

 body, called " thorax," which is partitioned off from the abdomen by 

 a transverse vaulted muscular flooi-, called the " diaphragm," {fig. 1. 

 d). Neither the circulation nor the respiration are quite so active, 

 nor is the animal heat so high, as in the class of Birds : few, indeed, 

 of the Mammalia enjoy the power of flight : most of the class are 

 quadrupeds, as they are commonly called -par excellence, and support 

 themselves and move by the action of four feet upon the ground. 

 Some burrow : most can swim, and a few are exclusively adapted for 

 living in water, and have the form of fishes ; in these the hinder 

 limbs are wanting, and the anterior ones present the shape of fins : 

 but all Mammalia breathe the air directly. The colouring particles 

 of the blood are more minute than in birds, and for the most part of 

 a circular form, i^fig- 4, a, b). 



With a few exceptions the jaws of the Mammalia are ai'med 

 with teeth, variously modified in subserviency to the habits and 

 food of the different species. In like manner the stomach is sim- 

 ple or complex, in relation to the amount of change to be effected 

 in the assimilation of the food : the small intestine is usually divided 

 from the large by the presence of a single cojcum, and the rec- 

 tum, with veiy few exceptions, has its outlet distinct from that 

 of the genital and urinary systems. The kidneys are supplied with 

 blood from the renal arteries exclusively ; but the liver continues 

 to receive the superadded system of the vena portte. The secretion 

 of the kidneys is always conveyed to a urinary bladder {fig- 1 . u), and 

 the penis is traversed by the urethral canal. 



All Mammalia intromit in fecundation, and all are viviparous : in 

 most the ovum, after quitting the ovary, becomes a second time at- 

 tached to the parent, through a variously modified cellulo-vascular 

 organ called " placenta." The young arc nourished after bii'tli by the 

 secretion of glands, called mammary — whence the name of the class. 

 Pei'haps the peculiar and constant existence of a well-developed 

 epiglottis, which Aristotle in one of his surprising generalisations 

 states to be present in all hairy viviparous quadrupeds, may have its 

 true relation of physiological co-existence with the mammary glands, 

 being most essential as a defence of the glottis during the act of 

 sucking. 



The bodies of the vertebrce are united to each other by concen- 

 tric ligaments, attached commonly to flattened surfaces, forming the 

 intervertebral substance. The cervical vertebras are seven, not varying 

 in number according to the length or sliortness of the neck. Hair 

 is the charactei'istic clothing of the body ; but almost all the modifi- 

 cations of the epidermal system which we meet with in the inferior 

 vertebrated classes are repeated in the present : thus we have quills 



